Normal sleep development in infants: findings from two large birth cohorts

1 Normal sleep development in infants: findings from two large birth cohorts E. Juulia Paavonen1,2, Outi Saarenpää-Heikkilä3, Isabel Morales-Munoz2,4, Minna Virta5, Niina Häkälä2, Pirjo Pölkki6, Anneli Kylliäinen7, Hasse Karlsson5,8,11, Tiina Paunio,2,9, Linnea Karlsson5,10,11 1. Paediatric Research Center, Child Psychiatry, University of Helsinki and Helsinki University Hospital, Helsinki, Finland 2. Finnish Institute for Health and Welfare, Helsinki, Finland 3. Center for Child Health Research, Tampere University, Faculty of Medicine and Health Technology and Tampere University Hospital 4. Institute for Mental Health, School of Psychology, University of Birmingham, Birmingham, United Kingdom 5. The FinnBrain Birth Cohort Study, Turku Brain and Mind Center, Institute of Clinical Medicine, University of Turku 6. Department of Social Sciences, University of Eastern Finland, Kuopio, Finland 7. Psychology, Faculty of Social Sciences, Tampere University, Tampere, Finland 8. Department of Psychiatry, Turku University Hospital and University of Turku 9. Department of Psychiatry and SleepWell Research Program, Faculty of Medicine, University of Helsinki and Helsinki University Hospital, Helsinki, Finland 10. Department of Child Psychiatry, Turku University Hospital and University of Turku, Finland 11. Centre for Population Health Research, University of Turku and Turku University Hospital ORCID ID’s: Juulia Paavonen: 0000-0002-1421-9877 Outi Saarenpää-Heikkilä: 0000-0002-5382-5888 Isabel Morales-Munoz: 0000-0002-4718-6768 Pirjo Pölkki: 0000-0003-3489-8747 Anneli Kylliäinen: 0000-0002-8839-3720 Hasse Karlsson: 0000-0002-4992-1893 Tiina Paunio: 0000-0002-5560-0666 Linnea Karlsson: 0000-0002-4725-0176 This is the accepted manuscript of the article, which has been published in Sleep Medicine. 2020, 69, 145-154. https://doi.org/10.1016/j.sleep.2020.01.009 Normal sleep in infants 2 Abstract Objective: Sleep difficulties are highly prevalent and often persistent in young children, but sometimes parents are worried about sleep symptoms that belong to the normative range rather than to actual disturbances. Therefore, the aim of this study was to describe the normative development of sleep at the ages of three, six, eight, 12, 18 and 24 months in healthy children. Methods: The present study is based on two birth cohorts that comprise representative samples of families recruited systematically during pregnancy. In the CHILD-SLEEP cohort, the sample sizes were 1427 at three, 1301 at eight, 1163 at 18, and 950 at 24 months. In the Finnbrain cohort, the sample sizes were 2002 at six months and 1693 at 12 months. Healthy term-born children were eligible for this study. To assess the infants’ sleep duration and sleep quality, the Brief Infant Sleep Questionnaire (BISQ) was used in both cohorts and additionally the Infant Sleep Questionnaire (ISQ) in the CHILD-SLEEP cohort. The distributions of the study variables were reported using standard parameters. Results: We found that sleep quality is highly variable particularly during the first two years of life, but this variability decreased markedly towards the second year. First, sleep latency decreased by the age of 6 months, while night-time sleep began to consolidate during the second year. However, parent-reported sleeping problems were common during the entire study period. Conclusion: As many families struggle with infants’ sleeping problems, the reference values reported in this article can be valuable tools in various clinical settings to define clinically significant deviances in the sleep development and to identify individuals benefitting from counselling and clinical interventions. KEYWORDS: Sleep, infants, development, normal sleep, sleep duration, sleep quality Normal sleep in infants 3 Introduction Sleep difficulties are highly prevalent and often persistent in young children (Fricke-Oerkermann et al., 2007; Williams et al., 2019). They tend to be problematic for the entire family by interrupting the sleep of the other family members. Therefore parents, often seek advice from healthcare professionals to help to solve these problems. However, inter-individual differences in sleep development are large and result in highly variable sleep quality (Iglowstein et al. 2003; Sadeh et al. 2009); sometimes, parents are worried about sleep symptoms that belong to the normative range rather than representing actual disturbances requiring clinical assessment and treatment. The extensive changes in sleep development during the first two years concern three different dimensions that form the basis of quality sleep. First, sleep-wake rhythms start to mature soon after birth. Among new-born infants, sleep is distributed irregularly during the day and night, but the rhythmicity evolves within a few months so that usually at around three months of age clear signs of a diurnal sleep-wake rhythm can be seen. This development is reflected as more sleep during the night and less sleep during the day (Rivkees, 2003). Second, infants’ self-regulation starts to develop during the first year of life (Rothbart et al., 2011), which leads to an improved ability to fall asleep independently. This is one key element in the development of the self-regulated sleep-wake cycle (Burnham et al., 2002) and together with the maturing sleep-wake rhythms they lead to the consolidation of night-time sleep and a reduced number of night awakenings (Hendersson et al., 2012). Finally, sleep needs decrease gradually from birth until adolescence, the decrease being at its fastest during the first two years of age (Iglowstein et al., 2003, Williams et al. 2013). Age-specific reference values are valuable tools in clinical decision making, in order to differentiate clinically significant deviances from normal development. To date, there are several studies concerning the average sleep duration in early childhood (Iglowstein et al. 2003; Montgomery-Downs et al. 2006; Williams et al. 2013; Xiao-Na et al., 2009) and clinical recommendations for the daily sleeping time in children aged 0-17 years. For example, the National Sleep Foundation recommends 14-17 hours of sleep for new-borns (0-3 months), 12-15 hours for infants (4-11 months) and 11-14 hours for toddlers (1-3 years) (Hirskowitz et al. 2015). The following recommendations are given by the American Academy of Sleep Medicine: 12-16 hours per 24 hours for infants aged 4-12 months and 11-14 hours for children aged 1-2 years (Paruthi et al. 2019). However, less prior research exists concerning the developmental changes in sleep quality, sleepwake rhythms or self-soothing. Two existing meta-analyses on sleep development in childhood Normal sleep in infants 4 identified only a few studies regarding infancy and early childhood (Dias, Figueiredo, Rocha, & Field, 2018; Galland, Taylor, Elder, & Herbison, 2012). For instance, Galland et al. (2012) found only four studies concerning sleep-duration in children aged 3-5 months and nine studies of 1-2 year olds. These studies were mainly based on small samples, comprising 20-56 cases (Harrison et al. 2004; Spruyt et al. 2008) or non-random sampling (Sadeh et al. 2009), which can result in selection bias and overreporting of sleeping problems. Moreover, according to this meta-analysis, no longitudinal studies so far have covered both the sleep duration and sleep quality. Therefore, new studies are necessary to define the boundaries of normative sleep patterns in early childhood. To ensure the adequate diagnostics and treatment of children’s sleeping difficulties, it would be important 1) to screen for sleep disturbances in well-baby clinics, 2) to provide preventive interventions for parents with concerns about sleep development 3) to enhance the availability of behavioural sleep interventions for children with sleeping disorders and 4) to increase the understanding of normal sleep development in children. However, it has been widely recognised that there are significant gaps among clinicians in knowledge concerning paediatric sleep disorders and that few children receive evidence-based care (Honaker & Meltzer, 2015; Owens, 2011). The barriers to their provision were studied recently and inadequate knowledge, training and education on children’s sleeping difficulties were the most commonly reported factors (Boerner et al. 2015). One solution to this problem could be the development of effective screening questions and tools for primary care (Honaker & Meltzer et al. 2015). Therefore, in this study we describe the normative development of sleep at the ages of three, six, eight, 12, 18 and 24 months in healthy children based on two large birth cohorts, in Finland. More specifically, we evaluate the normative ranges in the daytime, night-time and total sleep duration, the development of the sleep-wake rhythm and sleep quality represented by sleep latency, the number of night awakenings, number of signalled night awakenings and the time spent awake at night. We also report the number of naps and the percentage of children being able to self-sooth. We also suggest cut-off points at six time-points to be used in clinical settings to recognise deviant development. The normative parameters reported in this article can be used as reference values to define the normative range for children aged 0-2 years in various clinical settings, such as well-baby clinics. Normal sleep in infants 5 Methods The present study is based on two population-based cohorts in Finland: the CHILD-SLEEP (CS) and the FinnBrain (FB) cohorts. Both are based on representative samples recruited systematically during pregnancy. They are completely independent samples with no overlapping individuals. Both samples represent Urban/Semi-Urban areas in Southern and Western Finland with population of highly respective cultural backgrounds. Health services are similar in both areas and socio-economic differences are generally small in Finland. Both study protocols were approved by the local ethical committees. The CS cohort was recruited during pregnancy at maternity clinics at about week 32 of pregnancy. The sample comprised Finnish-speaking women who belonged to the area of the Tampere University Hospital, in the hospital district of Pirkanmaa, in southern Finland. The recruitment took place during a routine visit to the maternity clinics in the 32nd week of pregnancy between April 2011 and January 2013. The details of the recruitment procedure have been reported previously (Paavonen et al., 2017). Altogether, 1679 families agreed to initially participate in the study, which is approximately 25% of the target population. The response rates of participation were 84.6% (N=1427) at the age three months, 77.8% (N=1301) at eight months, 69.5% (N=1163) at the age of 18 months and 56.8% (N=950) at the age of 24 months. The FB cohort consists of another population-based sample gathered in southwest Finland (www.finnbrain.fi). The characteristics of the sample and more detailed description of the recruitment process have been described elsewhere (Karlsson et al., 2018). Briefly, the recruitment of the baseline population took place between December 2011 and April 2015 at the maternal welfare clinics in a geographically defined area, which performed pregnancy ultrasound scans at gestational week 12 for the women eventually referred to give birth at Turku University Hospital in the Southwest Finland Hospital District and the Aland Islands in Finland. The study inclusion criteria were: a sufficient knowledge of Finnish or Swedish and a normal ultrasound screening result. In all, 5790 out of 8895 newly pregnant women visiting the recruitment sites during the specified time period were contacted and informed of the study. Of those informed of the study, a total of N=3808 (66%) mothers and N=2623 (45%) fathers or other partners of the mother decided to participate. Finally, the response rates were 52.6% (N=2002) at the age of 6 months and 44.5% (N=1693) at the age of 12 months. This study comprises healthy term-born infants in both cohorts. Children who were born at the gestational age of 37 weeks or more (97.7%, N=1464 in the CS and 93.1%, N= 3545 in the FB) were eligible for this study. In the CS cohort, infants with missing information (N=41) on their gestational Normal sleep in infants 6 age were also considered eligible, as most of the infants were full term (≤36 N=35). Children with severe parent-reported chronic illnesses were excluded. The reported illnesses were reviewed by two paediatricians to confirm similar exclusion criteria in both cohorts. Decisions on this aspect were made by consensus. Examples of the excluded illnesses include epileptic syndromes, Down syndrome, Crouzon syndrome, Hirschprung disease, or hereditary blindness, and severe congenital heart anomalies such as Tetralogy of Fallot. In total, the number of excluded cases came to 14 cases in the CS cohort and 13 cases in the FB cohort. In the CS, altogether 173 children participated in a prevention trial, where they received written information from the health-care nurses about children’s sleep development. These children were excluded from the study. Finally, some of the questionnaires were returned late and were therefore excluded from the final analyses. Infants aged 3-4 months (N=1132, 47.8% girls, 52.2% boys), 6-7 months (N=1833, 47.2% girls, 52.8% boys), 8-9 months (N=1102, 47.9% girls, 52.1% boys), 11-15 months (N=1538, 46.5% girls, 53.5% boys), 17-20 months (N=848, 48.1% girls, 51.9% boys), and 22-28 months (N=771, 47.0% girls, 53.0% boys) were eligible for the study. In the drop-out analysis, we found that in the CS mothers participating at 24 months were older than those who did not (p<0.001), had higher educational levels (p<0.001), as well as lower levels of anxiety (p=0.002), depression (p=0.001), and ADHD symptoms (p=0.005). In the FB, the responding mothers were at 12 months more educated (p<0.001), had lower number of children (p<0.001), and less depressive symptoms (p<0.001). However, the group differences were small and therefore we present the findings without performing stochastic regression imputation. Methods This study was questionnaire-based. In both cohorts, the questionnaires were sent to both parents to be filled out by either one of the parents or both of them together. To assess the infants’ sleep duration and sleep quality, we used the Brief Infant Sleep Questionnaire (BISQ) in both cohorts and additionally the Infant Sleep Questionnaire (ISQ) in the CS cohort. The BISQ is a 13-item parental questionnaire for screening infant and toddler sleeping problems (Sadeh, 2004). This questionnaire comprises twelve items about sleep duration, sleep latency (“how long does it take to put the child to sleep”), and number and duration of night awakenings. In addition, there are items about bedtime, falling asleep and the preferred sleeping position. The ISQ is another parental questionnaire to evaluate the sleep quality of infants and toddlers (Morrell, 1999). There are altogether seven items related to difficulties getting to sleep, the frequency Normal sleep in infants 7 of night awakenings, and sleep onset problems in the evenings and at night. We used those items that were considered to provide complementary information relative to the BISQ. These items concerned the frequency of sleep onset problems and night awakenings (i.e., how many times per week they appeared), how often the infant required settling back to sleep and how often the infants were taken into their parents’ beds because they were not able to fall asleep. There were six to eight response alternatives for the items, but to indicate clinically relevant difficulties the items were dichotomized (the original items and the cut-offs are listed in Table 3). The parents were also asked whether they considered their child to be having sleep difficulties. We also report the number of naps at the ages of eight, 18 and 24 months based on the CS cohort. The parents’ were asked “How many naps does your baby take between 8-20?” with the response alternatives 0, 1, 2, and three or more. Furthermore, the parents were also asked about self-soothing at three, eight, 18 and 24 months. The item was “How often does your baby fall asleep independently without the parent being present?” with the response options being “never”, “rarely (less than once a week)”, “sometimes (several times per week)”, “often (daily at least once)” and “always”. This was dichotomized at often/always vs. less to indicate ability of the infant to self-sooth. The sleep data was carefully screened to identify extreme outliers. Potential errors were excluded. Both daytime and night-time sleep durations were required to be reported at the age of 0-1 years, but thereafter reporting no daytime sleep was considered acceptable. The accepted range for the total sleep duration was 6-21 hours at all time points. There were three excluded cases at the age of three months in the CS cohort and two cases at the age of six months and two at the age of 12 months in the FB cohort. There were no exclusion criteria for other sleep variables. Sleep parameters The studied sleep variables were: 1) the total sleep duration during the daytime, night-time and total sleep per 24 hours in hours (BISQ); 2) the sleep onset time in the evening in minutes (BISQ) and sleep onset difficulties at night (ISQ) (sleep latency ≥ 20 minutes during the night); 3) night awakenings: the average number of night awakenings between 22-06 (BISQ), the average number of night awakenings per week (5-7 nights a week vs. less) (ISQ), and the number of night awakenings between 00-06 per night when the child requires re-settling (ISQ) (≥2 times per night or ≥ 4 times per night vs less); 4) parents’ reports of the child having sleep problems (none, mild, severe) based on the BISQ and (none, mild, moderate, severe) based on the ISQ; 5) whether the child falls asleep independently (BISQ), how often the child falls asleep independently (own item); and 6) the number Normal sleep in infants 8 of naps (own item). The cut-off values for ISQ variables were based on clinical expertise to indicate problems that might be clinically relevant. Statistical methods The distributions of the study variables were reported using standard parameters. Means and standard deviations, minimum and maximum values and the 10th, 25th, 75th and 90th percentiles were reported. The percentages and their 95% confidence intervals (CIs) were reported for the categorical variables. The skewness of the distributions of the continuous variables was visually inspected. The differences between girls and boys were compared using either the t-test (daytime, night-time, total sleeping time, proportion of daytime sleep, and bedtime) or the Kruskal-Wallis test (sleep onset latency, number of night awakenings, time awake at night) depending on the distribution of the variable. Significant group differences were reported. Results Characteristics of the two samples are reported in Table 1. As reported in Table 2, the average total sleep duration decreased from 14.4 h at the age of three months to 11.9 h at the age of two years (Figure 1). The range was very high particularly in early childhood, but it decreased markedly during the follow up. While the nocturnal sleeping time (19-07) remained stable during the two years being about 9.1 – 10.2 h per night, the daytime sleep (07-19) decreased from 5.3 h at three months to 1.9 h at two years (Figures 2-3). Consistent with this, the proportion of daytime sleep decreased from 36.5 % for three month olds to 15.7 % for two year olds (Figure 4). The number of naps decreased from two at the age of eight months to one at the age of two years. More specifically, at eight months most of the children had two (71.6%, N=786) or three or more naps (23.7%, N=260), while only 4.7% (N=52) had just one nap. At 18 months, almost all children were reported to have only one nap per day (97.8%, N=829), while having two naps (2.0%, N=17) or no naps at all (0.2%, N=2) was rare. By the age of two years, the number of children without naps increased slightly to 2.7% (N=21). However, the majority were still napping once a day (97.0%, N=746). Only two children (0.3%, N=2) took naps twice a day. The ability to self-soothe was relatively uncommon in our samples throughout the follow-up; the highest rate was seen at the age of 18 months, when it reached 50.2% (Table 2). Normal sleep in infants 9 The average sleep onset decreased during the first year from 38 to 22 minutes remaining on a similar level thereafter (Table 2). However, yet about 25% of the children were still reported to have long sleep-onset latencies, exceeding 30 minutes (Table 3). Night awakenings were common at all time points. The average number of night awakenings was 2.1-2.5 during the first year but it decreased to an average of 1.1 awakenings per night in 18 month olds and 0.9 in two year olds (Table 2). However, the range was large being, for example, at eight months from zero to 21. Moreover, in two year olds, 28.4% of the children still woke up every night or almost every night between 00-06 and 14.9% had at least two night awakenings for which they needed resettling (Table 3). However, they rarely required resettling several times a night; the rates for ≥4 per night ranged from 13.6% to 2.1% (Table 3). Importantly, even though children often woke up during the night, sleep onset difficulties at night were less common. On average, the total time spent awake at night (between 00-06) decreased with age, from 24 minutes at eight months to six minutes at two years (Table 2). Moreover, prolonged sleep onset latency at night (≥20 minutes) decreased respectively, from 16.9% at three months to 2.2% at two years (Table 3). In Figure 5, we report the proposed cut-off values for sleep latency, night awakenings and the time spent awake at night based on the 10th, 25th, 75th and 90th percentiles at all time-points. In this figure the cut-offs, coloured orange (to be evaluated in more detail) and red (clinical assessment recommended), could be useful for screening clinically significant cases. Infants’ exceeding the 75th or 90th percentile have worse sleep quality than most of the other infants (75% or 90% of the infants, respectively) and they may suffer from clinically relevant sleep disturbances. The parent reported sleeping problems were common with rates ranging from 21.9% (at 3 months) to 39.7% (at 8 months) in the CS cohort and 27.8% (12 months) to 30.7% (6 months) in the FB cohort. Severe problems were reported only for a minority of children (in 0.6% to 1.1% in the CS cohort and 1.9% and 2.2% in the FB cohort) (Figure 6). The only sex differences we found across the time points were that: 1) girls slept more during the day than the boys both at three (5.4 h vs. 5.1 h, p=0.006), and eight months (p=0.017). 2) They also had a longer sleep onset latency than the boys at three months (40.4 vs. 35.3 min, pKW=0.005) while the boys had more night awakenings at three months than the girls (2.3 vs. 2.1, pKW<0.001). Normal sleep in infants 16 References 1. Akacem, L. D., Simpkin, C. T., Carskadon, M. A., Wright, K. P., Jenni, O. G., Achermann, P., LeBourgeois, M.K. The Timing of the Circadian Clock and Sleep Differ between Napping and Non-Napping Toddlers. PLoS One 2015; 10(4): e0125181. 2. Allen, S. L., Howlett, M. D., Coulombe, J. A., Corkum, P. V. ABCs of SLEEPING: A review of the evidence behind pediatric sleep practice recommendations. Sleep Med Rev. 2016; 29: 1-14. doi: 10.1016/j.smrv.2015.08.006. 3. Anders, T. F., Halpern, L. F., Hua, J. Sleeping through the night: a developmental perspective. Pediatrics. 1992; 90(4):554-60. 4. Boerner, K. E., Coulombe, J. A. Corkum, P. Barriers and facilitators of evidence-based practice in pediatric behavioral sleep care: qualitative analysis of the perspectives of health professionals. Behav Sleep Med. 2015; 13(1): 36-51. 5. Borbély, A.A., Daan, S., Wirz-Justice, A., Deboer, T. The two-process model of sleep regulation: a reappraisal. J Sleep Res.2016; 25(2):131-43. doi: 10.1111/jsr.12371. 6. Brescianini, S., Volzone, A., Fagnani, C., Patriarca, V., Grimaldi, V., Lanni, R., Serino, L., Mastroiacovo, P., Stazi, M. A. Genetic and environmental factors shape infant sleep patterns: a study of 18-month-old twins. Pediatrics. 2011;127(5):e1296-302. doi: 10.1542/peds.20100858. 7. Bruni, O., Baumgartner, E. Sette, S., Ancona, M., Caso, G., Di Cosimo, M.E., Mannini, A., Ometto, M., Pasquini, A., Ulliana, A. Ferri, R. Longitudinal Study of Sleep behavior in Normal Infants during the First Year of Life. J Clin Sleep Med. 2014; 10(10): 1119-1127. 8. Burnham, M. M., Goodlin-Jones, B. L., Gaylor, E., Anders, T. F. Nighttime sleep-wake patterns and self-soothing from birth to one year of age: a longitudinal intervention study. J Child Adol Psychol Psy 2002; 43(6): 713-725. 9. Chaput, J-P., Gray, C. E., Poitras, V. J., Carson, V., Gruber, R., Birken, C. S., MacLean, J. E., Aubert, S., Sampson, M., Tremblay, M. S. Systematic review of the relationships between sleep duration and health indicators in the early years (0–4 years). BMC Public Health. 2017; 17(Suppl 5): 855. doi: 10.1186/s12889-017-4850-2 10. de Ruiter I, Olmedo-Requena, R., Sánchez-Cruz, J. J., Jiménez-Moleón, J. J. Changes in sleep duration in Spanish children aged 2-14 years from 1987 to 2011. Sleep Med. 2016; 21:14550. doi: 10.1016/j.sleep.2015.12.021. 11. Dias, C. C., Figueiredo, B., Rocha, M., Field, T. Reference values and changes in infant sleepwake behavior during the first 12 months of life: a systematic review. J Sleep Res. 2018; 27: 1-23. 12. Ednick, M., Cohen, A. P., McPhail, G. L., Beebe, D., Simakajornboon, N., Amin, R. S. A Review of the Effects of Sleep During the First Year of Life on Cognitive, Psychomotor, and Temperament Development. Sleep 2009; 32(11): 1449–1458. 13. Felt, B.T., Chervin, R. D. Medications for sleep disturbances in children. Neurol Clin Pract 2014; 4(1): 82–87. 14. Fisher, A., Llewellyn, C. H., Wardle, J. Genetic and environmental influences on infant sleep. Pediatrics. 2012; 129(6):1091-6. doi: 10.1542/peds.2011-1571. 15. Fricke-Oerkermann, L., Plück, J., Schredl, M., Heinz, K., Mitschke, A., Wiater, A., Lehmkuhl, G. Prevalence and course of sleep problems in childhood. Sleep 2007; 30(10):1371-1377. 16. Galland, B.C., Taylor, B.J., Elder, D.E., Herbison, P. Normal sleep patterns in infants and children: A systematic review. Sleep Med Rev 2012; 16: 213-222. 17. Goodlin-Jones, B. L., Burnham, M. M., Gaylor, E.E., Anders, T.F. Night waking, sleep-wake organization, and self-soothing in the first year of life. J Dev Behav Pediatr. 2001; 22(4):22633. Normal sleep in infants 17 18. Harrison, Y. The relationship between daytime exposure to light and night-time sleep in 612-week-old infants. J Sleep Res. 2004;13(4):345-52. 19. Hirshkowitz, M., Whiton, K., Albert, S. M., Alessi, C., Bruni, O., DonCarlos, L., Hazen, N., Herman, J., Adams Hillard, P.J., Katz, E.S., Kheirandish-Gozal, L., Neubauer, D. N., O'Donnell, A.E., Ohayon, M., Peever, J., Rawding, R., Sachdeva, R.C., Setters, B., Vitiello, M.V., Ware, J.C. National Sleep Foundation's updated sleep duration recommendations: final report. Sleep Health. 2015;1(4):233-243. doi: 10.1016/j.sleh.2015.10.004. 20. Jenni, O. G., O'Connor, B. B. Children's sleep: an interplay between culture and biology. Pediatrics.2005;115 (1 Suppl):204-16. 21. Katila, M., Saarenpää-Heikkilä, O., Saha, M.T., Vuorela, N., Paavonen E.J. Parental reports showed that snoring in infants at three and eight months associated with snoring parents and smoking mothers. Acta Paediatr. 2019; 108(9):1686-1694. doi: 10.1111/apa.14758. 22. McDonald, L., Wardle, J. Llewellyn, C. H., van Jaarsveld, C. H. M., Fisher, A. Predictors of shorter sleep in early childhood. Sleep Medicine, 2014; 15: 536-540. Doi: 10.1016/j.sleep.2014.01.005. 23. Meltzer, L. J. Clinical Management of Behavioural Insomnia of Childhood: Treatment of Bedtime Problems and Night Wakings in Young Children. Behav Sleep Med, 2010; 8: 172189. 24. Mindell, J. A., Kuhn, B., Lewin, D. S., Meltzer, L. J., Sadeh, A. Behavioral treatment of bedtime problems and night wakings in infants and young children – an American Academy of Sleep Medicine review. Sleep. 2006; 29: 1263-76. 25. Mindell, J. A. Meltzer, L. J. Carskadon, M. A., Chervin, R. D. Developmental aspects of sleep hygiene: Findings from the 2004 National Sleep Foundation Sleep in America Poll. Sleep Med, 2009, 10: 771-779. 26. Mindell, J. A., Du Mond, C. E., Sadeh, A., Telfski, L. S., Kulkami, N., Gunn, E. Long-term Efficacy of an Internet-based Intervention for Infant and Toddler Sleep Disturbances: One Year Follow-Up. J Clin Sleep Med, 2011, 7(5): 507-511. 27. Montgomery-Downs, H. E., O'Brien, L. M., Gulliver, T. E., Gozal, D. Polysomnographic characteristics in normal preschool and early school-aged children. Pediatrics. 2006;117(3):741-53. 28. Morales-Munoz, I., Saarenpää-Heikkilä, O., Kylliäinen, A., Pölkki, P., Porkka-Heiskanen, T., Paunio, T., Paavonen, E. J. The effects maternal risk factors during pregnancy on the onset of sleep difficulties in infants at 3 months old. J Sleep Res, 2018, 27(5):e12696. Doi:10.1111/jsr.12696. 29. Morales-Muñoz, I., Partonen, T., Saarenpää-Heikkilä, O., Kylliäinen, A., Pölkki, P., PorkkaHeiskanen, T., Paunio, T., Paavonen, E. J. The role of parental circadian preference in the onset of sleep difficulties in early childhood. Sleep Med. 2019; 54: 223-230. doi: 10.1016/j.sleep.2018.10.039. 30. Morsbach Honaker, S., Melzer, L. J. Sleep in pediatric primary care: A review of the literature. Sleep Med Revs. 2016; 25: 31-39. 31. Moturi, S., Avis, K. Assessment and treatment of common pediatric sleep disorders. Psychiatry (Edgmont) 2010; 7(6): 24–37. 32. Mäkelä, T. E., Peltola, M.J., Nieminen, P., Paavonen, E. J., Saarenpää-Heikkilä, O., Paunio, T., Kylliäinen, A. Night awakening in infancy: Developmental stability and longitudinal associations with psychomotor development. Developmental Psychology, 2018, 54(7):12081218. Doi: 10.1037/dev0000503. 33. Nevarez, M. D., Rifas-Shiman, S.L., Kleinman, K. P., Gillman, M.W. Taveras, E. M. Assocations of Early Life Risk Factors With Infant Sleep Duration. Academic Pediatrics. 2010: 10: 187-93. Normal sleep in infants 18 34. O'Connor, T. G., Caprariello, P., Blackmore, E. R., Gregory, A. M., Glover, V., Fleming, P.; ALSPAC Study Team. Prenatal mood disturbance predicts sleep problems in infancy and toddlerhood. Early Hum Dev. 2007; 83(7):451-8. 35. Paavonen, E. J., Morales-Muñoz, I., Pölkki, P., Paunio, T., Porkka-Heiskanen, T., Kylliäinen, A., Partonen, T., Saarenpää-Heikkilä, O. Development of sleep-wake rhythms during the first year of age. J Sleep Res, 2019; e12918. Doi: 10.1111/jsr.12918 36. Paruthi, S., Brooks, L. J., D'Ambrosio, C., Hall, W. A., Kotagal, S., Lloyd, R. M., Malow, B. A., Maski, K., Nichols, C., Quan, S. F., Rosen, C. L., Troester, M. M., Wise, M. S. Consensus Statement of the American Academy of Sleep Medicine on the Recommended Amount of Sleep for Healthy Children: Methodology and Discussion. J Clin Sleep Med. 2016;12(11):1549-1561. 37. Pennestri, M. H., Laganière, C.,Bouvette-Turcot, A. A.,Pokhvisneva, I.,Steiner, M., Meaney, M.J.,Gaudreau, H.,Mavan Research Team. Uninterrupted Infant Sleep, Development, and Maternal Mood. Pediatrics. 2018;142(6). pii: e20174330. doi: 10.1542/peds.2017-4330. 38. Rivkees, S. A. Developing circadian rhythmicity in infants. Pediatrics. 2003;112(2):373-81. 39. Rothbart, M.K., Sheese, B. E., Rueda, M. R., Posner, M. I. Developing Mechanisms of SelfRegulation in Early Life. Emotion Review. 2011 Apr; 3(2): 207–213. doi:10.1177/1754073910387943 40. Sadeh A, Mindell J.A., Luedtke, K., Wiegand, B. Sleep and sleep ecology in the first three years: a web-based study. J Sleep Res 2009, 18: 60-73. 41. Sadeh, A., Tikotzky, L., Scher, A. Parenting and infant sleep. Sleep Med Rev. 2010; 14(2): 89-96. doi: 10.1016/j.smrv.2009.05.003. 42. Spruyt, K. A review of developmental consequences of poor sleep in childhood. Sleep Med, 2019, 60: 3-12. 43. Spruyt, K. Aitken, R. J., So, K., Charlton, M., Adamson, T. M., Horne, R. S. Relationships between sleep/wake patterns, temperament and overall development in term infants over the first year of life. Early Hum Dev. 2008; 84: 289-96. 44. Tham, E. K. H., Schneider, S., Broekman, B. F. P. Infant sleep and its relation with cognition and growth: a narrative review. Nat Sci Sleep 2017; 9: 135–149. 45. Thiedke, C. C. Sleep disorders and sleep problems in childhood. Am Fam Physician. 2001; 15;63(2):277-84. 46. Thumann, B. F., Börnhorst, C., Michels, N., Veidebaum, T., Solea, A., Reisch, L., Moreno, L.A., Lauria, F, Kaprio, J., Hunsberger, M., Felső, R,. Gwozdz, W., De Henauw, S.., Ahrens, W. IDEFICS and I. Family consortia. Cross-sectional and longitudinal associations between psychosocial well-being and sleep in European children and adolescents. J Sleep Res. 2019; 28(2):e12783. doi: 10.1111/jsr.12783. 47. Touchette, E., Dionne, G., Forget-Dubois, N., Petit, D., Pérusse, D., Falissard, B., Tremblay, R. E., Boivin, M., Montplaisir, J. Y. Genetic and environmental influences on daytime and nighttime sleep duration in early childhood. Pediatrics. 2013;131(6):e1874-80. doi: 10.1542/peds.2012-2284. 48. Turnbull, K., Reid, G.J., Morton, J.B. Behavioral Sleep Problems and their Potential Impact on Developing Executive Function in Children. Sleep 2013; 36(7): 1077–1084. 49. Vriend, J., Corkum, P. Clinical management of behavioral insomnia of childhood. Psychol Res Behav Manag 2011; 4: 69–79. 50. Wake, M., Morton-Allen, E., Poulakis, Z., Hiscock, H., Gallagher, S., Oberklaid, F. Prevalence, stability, and outcomes of cry-fuss and sleep problems in the first 2 years of life: prospective community-based study. Pediatrics. 2006; 117(3): 836-42. 51. Weinraub, M., Bender, R., Friedman, S., Susman, E.J., Knoke, B., Bradley, R., Houts, R., Williams, J., McCartney, K. and the NICHD Early Child Care Research Network. Patterns of Normal sleep in infants 19 Developmental Change in Infants’ Nighttime Sleep Awakenings From Six through 36 Months of Age. Dev Psychol. 2012; 48(6): 1511-28. d: 10.1037/a0027680. 52. Williams, J.A., Zimmerman F.J., Bell, J.F. Norms and Trends of Sleep Time Among US Children and Adolescents. JAMA Pediatr 2013, 167(1): 55-60. 53. Williamsson, A. A., Mindell, J. A., Hiscock, H, Quach, J. Child sleep behaviours and sleep problems from infancy to school-age. Sleep Med, 2019, 63: 5-8. 54. Xiao-Na, H., Hui-Shan, W., Jing-Xiong, J., Yu-Yan, M., Lin, A., Xi-Cheng, L. The epidemiology of sleep and its disorder in Chinese children aged 0-5 years. Biol Rhythm Res. 2009; 40: 399-411. Normal sleep in infants 20 Table 1. Description of the two samples. CHILD-SLEEP1 FinnBrain2 Mean ± SD (Range) Mean ± SD (Range) Birth weight (g) in grams 3589 ± 447 (2290 – 5780) 3601 ± 467 (1780 – 5470) Gestational age (weeks) in weeks 40.1 ± 1.1 (37.0 – 42.6) 40.0 ± 1.2 (37.0 – 42.6) Maternal age in years 31.0 ± 4.3 (19 – 47) 30.7 ± 4.3 (18 – 45) Paternal age in years 33.1 ± 4.3 (20 – 57) 32.0 ± 5.3 (18 – 60) N (%) N (%) Number of children in the family one two three or more 498 (47.2%) 364 (34.5%) 193 (18.3%) 974 (53.2%) 611 (33.4%) 246 (13.4%) Breastfeeding full partial no 727 (64.5%) 254 (22.5%) 146 (13.0%) 149 (12.4%) 699 (58.3%) 350 (29.2%) 1. Data concerns the respondents at three months. 2. Data concerns the respondents at six months. Normal sleep in infants 21 Table 2. Normative sleep development in children aged 3 months to 2 years. 3 mo 6 mo 8 mo 12 mo 18 mo 24 mo mean ± SD (range) mean ± SD (range) mean ± SD (range) mean ± SD (range) mean ± SD (range) mean ± SD (range) Daytime sleep, hours 5.3 ± 1.5 (1.0-10.5) 3.7 ± 1.2 (0.5-9) 3.4 ± 1.0 (1.0-9.0) 2.5 ± 0.8 (0.5-6.0) 2.1 ± 0.7 (0.7-5.5) 1.9 ± 0.7 (0-5.0) Night sleep, hours 9.1 ± 1.4 (2.0-12) 9.9 ± 1.1 (2.5-12) 9.9 ± 1.0 (6.0-12.0) 10.2 ± 1.0 (3.5-12.0) 10.2 ± 0.8 (5.5-12.0) 10.0 ± 0.8 (6.8-12.0) Total sleep, hours 14.4 ± 1.8 (6-20.5) 13.7 ± 1.4 (6-21) 13.3 ± 1.2 (8.0-21.0) 12.8 ± 1.1 (6.5 – 16.0) 12.3 ± 0.9 (8.0-16.0) 11.9 ± 0.9 (8.3-14.5) 8-16 8 Daytime sleep, % 36.5 ± 8.4 (10.0-76.5) 27.0 ± 7.2 (4.6-50.0) 25.7 ± 6.5 (8.3-53.9) 19.6 ± 5.8 (4.8-46.2) 16.9 ± 4.8 (6.0-38.5) 15.7 ± 5.0 (0-40.0) Sleep onset latency, minutes 38.0 ± 34.3 (0-420) 25.4 ± 20.4 (0-180) 22.4 ± 16.3 (0-120) 21.7 ± 20.0 (0-300) 19.7 ± 14.2 (0-120) 25.4 ± 19.5 (0-210) Number of night awakenings 2.2 ± 1.3 (0-15) 2.5 ± 1.8 (0-15) 2.4 ± 1.8 (0-21.5) 1.8 ± 1.5 (0-13.5) 1.1 ± 1.1 (0-11) 0.9 ± 0.9 (0-6) Time awake at night, minutes 53.3 ± 50.2 (0-480) 27.3 ± 29.2 (0-300) 23.7 ± 25.7 (0-180) 16.8 ± 25.0 (0-300) 8.5 ± 22.7 (0-480) 6.1 ± 11.0 (0-120) Bedtime, hh:mm 21:46 ± 1:13 (18:00-02:10) 20:54 ± 1:00 (18:00-02:00) 20:53 ± 0:56 (18:20-02:00) 20:39 ± 0:50 (18:15-02:00) 20:41 ± 0:43 (18:30-23:00) 20:54 ± 0:41 (18:4523:30) Normal sleep in infants 22 Table 3. The prevalence of other sleeping problems and self-soothing 3 mo 8 mo 18 mo 24 mo Sleep quality Cut-off % (N) (95% CI) % (N) (95% CI) % (N) (95% CI) % (N) (95% CI) How many times a week do you have problems settling him/her on average? ≥ 5 nights a week 7.6% (6.1-9.2%) 12.0% (10.1-13.9%) 3.4% (2.2-4.6) 8.8% (6.8-10.8%) How many nights a week does your baby wake on average (between 24:00-06:00)? ≥ 5 nights a week 83.5% (81.3-85.7%) 77.7% (75.2-80.2%) 36.1% (32.9-39.3%) 28.4% (25.2-31.6%) How many times does your baby wake each night and need resettling on average? ≥ 2 times per night 48.8% (45.8-51.8%) 56.0% (53.0-59.0%) 22.0% (19.2-24.8%) 14.9% (12.4-17.4%) How many times does your baby wake each night and need resettling on average? ≥ 4 times per night 7.1% (5.6-8.6%) 13.6% (11.5-15.7%) 4.7% (3.3-6.1%) 1.8% (0.9-2.7%) If your baby wakes, how long does it take for your baby to go back to sleep on average? ≥ 20 min 16.9% (14.6-19.2%) 6.8% (5.3-8.3%) 4.8% (3.4-6.2%) 2.2% (1.1-3.3%) How often do you end up taking your baby into your bed because he/she is upset and won’t sleep? ≥ 5 nights a week 14.8% (12.6-17.0%) 25.3% (22.7-28.0%) 16.6% (14.1-19.1%) 11.1% (8.8-13.4%) How often does your baby wall asleep independently without the parent being present often/always 30.6% (27.9-33.3%) 49.4% (46.2-52.6%) 50.2% (46.8-53.6%) 46.7% (43.2-50.2%) 1RUPDOVOHHSLQLQIDQWV    Figure 1. Total sleep time in infants aged 3 to 24 months. 1RUPDOVOHHSLQLQIDQWV    Figure 2. Daytime sleep duration in infants aged 3 to 24 months.  Normal sleep in infants 25 Figure 3. Night-time sleep duration in infants aged 3 to 24 months. 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 2 4 6 8 10 12 14 16 18 20 22 24 Night-time sleep (h) Age (months) Avg -1SD +1SD -2SD +2SD