Our study identified several potential risk factors during the prenatal, infancy, and early childhood periods that are associated with infant sleep duration independent of sociodemographic characteristics. Shorter sleep duration was associated with maternal depression during pregnancy, introduction of solid foods prior to 4 months of age, infant TV/video viewing, and attendance at child care outside the home. Racial/ethnic minority children slept substantially less than their white counterparts in the first two years of life. At age 6 months, we also observed racial/ethnic differences in daytime nap and nighttime sleep duration. Whenever possible we examined longitudinal risk factors for sleep duration, but several of our infant risk factors were cross-sectional and therefore hold the potential for reverse causality (e.g., mothers whose infants are not sleeping well may be inclined to introduce solids earlier, and/or increase exposure to TV/video viewing). Our results, however, suggest that various risk factors, some potentially modifiable, are worthy of clinical consideration when addressing infant sleep duration.
Our finding that maternal antenatal depression is associated with shorter infant sleep duration is consistent with prior studies of maternal depression during pregnancy and infant sleep.15, 16
In the existing literature, as well in our analyses, maternal antenatal (rather than postpartum) depression has been found to be associated with negative impacts on infant sleep. In addition, this relationship to infant sleep supports the clinical relevance of monitoring perinatal maternal mood states. Although the mechanisms relating maternal prenatal mood disturbance and infant sleep have yet to be fully understood, it has been suggested that prenatal maternal anxiety and depression may lead to increased prenatal stress leading to elevated glucocorticoid secretion. In animal studies, elevated prenatal exposure to glucocorticoids has been shown to disrupt the circadian activity of the fetus' hypothalamic pituitary adrenal axis. Thus, it is possible that exposure to elevated prenatal exposure to glucocorticoids could disrupt infants’ sleep patterns.16
It is also possible that maternal depression can influence maternal behavior, which in turn can impact infant sleep patterns.
In the early infancy period, we found that introduction of solid foods prior to 4 months of age was associated with shorter sleep duration at ages 1 and 2 years. Although the American Academy of Pediatrics (AAP) recommends that solid foods be introduced to healthy children no earlier than 4 to 6 months of age, previous studies have found that parents believe solids help children sleep better at night, and it is one reason why they introduce solids before 4 months of age.17
To our knowledge, there are no published studies of timing of introduction of solids and infant sleep duration. Although previous studies have found that breastfeeding is associated with frequent night waking and shorter sleep duration during infancy, 18–21
we did not find an association between breastfeeding and infant sleep duration.
Consistent with previous studies of older children,9, 22, 23
we found that TV/video viewing was associated with shorter sleep duration at 1 and 2 years of age. Although the effect estimates of these associations are small, if the relationship between TV viewing and sleep loss track throughout childhood, the observed small magnitudes of effect could be additive throughout the child’s lifecourse. In this context of potential chronic sleep loss, even risk factors during infancy may be clinically relevant and merit awareness by clinicians. Numerous mechanisms have been proposed by which TV/video viewing may disturb children’s sleep including disruption of regular sleep schedules 9
and our findings support the AAP guidelines 24
of no TV viewing in infancy. We also observed a trend of shorter sleep duration among infants who attended child care outside of their home in the first two years of life. Child care outside of the home may present new challenges for infants, such as longer periods of parent-child separation that may influence their daily routine and sleep-wake schedules. It is also possible that parents might not be aware of the duration of naps during the day while at child care and could report fewer hours of total sleep duration. Infants might also have long naps while in child care which could shorten their nighttime sleep. Finally, we can not rule out the possibility of reverse causation, i.e. perhaps child care schedules required infants to be woken up earlier. To our knowledge, no previous studies have examined the relationship between child care attendance and infant sleep duration.
In unadjusted analyses, exposure to cigarette smoke, i.e. maternal smoking during pregnancy and at each time point measured in infancy, and total number of smoking household members, was associated with shorter sleep duration in infancy. However, in multivariable models adjusted for sociodemographic characteristics, the relationship between cigarette smoke exposure and sleep duration was attenuated. Previous studies have found an association of maternal prenatal smoking and smoking in early infancy with shorter sleep duration among infants. 11, 25
Our findings suggest that the observed association between smoking and sleep duration may be, in part, explained by parental sociodemographic characteristics.
We found substantial racial/ethnic differences in sleep duration in the first two years of life. Already by 6 months of age, black infants were sleeping almost 1 hour less than white children with significant differences in daytime nap and nighttime sleep durations. By age 2 years, sleep duration was shorter among black, Hispanic, and Asian children than among their white counterparts. Our findings persisted even after adjusting for potential confounders including sociodemographic characteristics. Our findings of racial/ethnic differences in daytime nap duration are consistent with those of older children.26, 27
In a study of 2- to 8-year-old children, Crosby et al.26
found that black children took daytime naps on more days per week and had shorter nighttime sleep duration than non-Hispanic white children. However, total weekly sleep duration was similar for the 2 racial groups. Among 2- to 5-year-old children, Lavigne et al. 27
found that racial/ethnic minority children napped longer and more often and spent less time sleeping at night than non-Hispanic white children, but also with comparable total sleep durations. The findings of our study differ in that less nighttime sleep among black children at 6 months of age was not balanced by longer daytime naps.
Viewed collectively with prior studies; our results suggest that racial/ethnic differences in sleep duration may extend to infancy. It is possible that the observed differences may reflect culturally distinct parental beliefs and practices regarding sleep. Previous studies have shown racial/ethnic differences in children’s bedtime routines, sleep location, and mothers’ level of concern about their child’s sleep.28
In addition, a study of 2- to 7-year-old children found African-Americans to have later bedtimes than whites with similar wake times, resulting in shorter sleep duration, independent of socioeconomic status. 29
Given the number of adverse health outcomes related to short sleep duration, including obesity and cardiovascular disease which are more prevalent in racial/ethnic minorities, our findings indicate a need to better understand the mechanisms relating race/ethnicity to sleep duration.
Our study had several strengths including the ability to examine a broad range of potential predictors and possible confounders in a large, pre-birth cohort of mother-infant pairs. Our study also had limitations. First, we measured sleep duration by mother’s report on the questionnaires as opposed to using an objective measure of sleep such as actigraphs or diaries. Although parental report has its limitations, a previous validation study among infants found that parental report of total sleep duration was significantly associated with infants having severe sleep problems, and was found to be a clinically useful measure to determine if a child needs subspecialty referral for sleep evaluation.30
In addition, potential bias exists for the reporting of sleep duration in our study. For example, antenatal depression could lead to postnatal depression and result in a systematic misreporting of infant sleep (~30% of our women who were depressed during pregnancy also reported being depressed at age 6 months and 1 year). Second, we did not measure infant sleep quality measures or other key sleep-related data, e.g. number of nighttime awakenings, sleep location/environment, bedtime routines, co-sleeping, crowding, or parental perceptions of sleep, all of which may be related to sleep duration. Third, although mothers in the study had diverse racial/ethnic backgrounds, their education and income levels were relatively high. Our results may not be generalizable to more socioeconomically disadvantaged populations. Finally, in any observational study it is possible that statistically significant associations may be surrogates for other risk factors and/or residual confounding may be present.