The results from this study provide support for the use of language-based bedtime routines as a sleep-promoting activity for disadvantaged preschoolers. The association between such routines and child sleep hours is robust, even after adjusting for a range of child, parent, and household characteristics.
One possible explanation for this finding may be found in recent research on infants and sleep, which indicates that maternal bedtime routines may not be as important for infant sleep quality as the maternal emotional availability that accompanies such routines (Teti, et al., 2010
). Maternal emotional availability is thought to allow children to feel safe in an environment that is “reasonably predictable, controllable, and free of potential threats” (Teti, et al., 2010
), and thereby promote sleep. While we are unable to adjust directly for maternal emotional availability in our study, we recognize that the parents who engage in language-based bedtime routines may either be more emotionally available to begin with, or may be interpreted by young children as being more emotionally available due to their regular use of such routines.
Another hypothesis is that the regular use of language-based bedtime routines, which tend to be time and attention intensive, are proxies for the quality of the overall home environment, such that more chaotic and less supportive home environments have lower rates of using language-based bedtime routines. To gain insight into this possibility, we tested whether there were mean differences in several home characteristics by whether a family used language-based bedtime routines. We found (results not shown) that families who used such routines were also more likely to put the focal child to bed at a regular time and were less punitive, more emotionally responsive, exhibited better verbal and social skills, and provided a greater level of language stimulation in the home. This suggests that language-based bedtime routines are likely to be used in concert with other family behaviors that are also associated with improved child sleep (and stimulate child development).
Turning to the developmental outcomes, we find a positive association between engaging in a language-based bedtime routine and better cognitive skills (PPVT-R) for preschoolers which, though attenuated with the addition of child, caregiver, and family characteristics, remains statistically significant even when such characteristics are included in our regression models. The magnitude of this association represents a 0.14 standard deviation difference in PPVT scores (about 2.2 points) at age 5 between children who did and did not experience language-based bedtime routines at age 3. To put the magnitude of this effect in context, the Head Start Impact Study (U.S. Department of Health and Human Services, 2010
) reports the intent-to-treat effect of one year of Head Start for randomly assigned 3 and 4 year olds, respectively, to be an increase of 0.18 and .09 standard deviations with regard to PPVT scores, relative to a control children. Thus, the effect size found in this study is slightly smaller than the average intent-to-treat effect of a randomly assigned year of Head Start for a 3 year old and about 50 percent larger than the average effect of a randomly assigned year of Head Start for a 4 year old.
This higher PPVT score among children engaging in regular language-based bedtime routines likely reflects the cognitive stimulation that occurs through the process of nightly engagement with a child through reading, storytelling, talking, singing, praying, and/or playing games. Our results also reveal that sleep duration does not fully mediate the association between language-based bedtime routine and higher PPVT-R scores. This suggests that improved sleep duration is probably not the sole mechanism explaining the higher scores, but rather that such activities directly stimulate literacy. Indeed, there is ample evidence that access and exposure to book sharing promotes positive cognitive development (Raikes, et al., 2006
; Rodriguez, et al., 2009
). Furthermore, as noted above, however, it is important to keep in mind that language-based bedtime routines are likely to co-occur with other language-stimulating behaviors.
Our findings with regard to behavior problems and general health indicate that associations between language-based bedtime routines and each of these outcomes are nonsignificant after adjusting for a range of confounding variables (e.g., child, caregiver, and household characteristics). That is, other characteristics that are likely associated with both bedtime routines and child development appear to be driving the associations we observe in our simplest unmatched models (Model 1). Whereas language-based bedtime routines may not be directly associated with child behavior, our findings show (), that there are statistically significant associations between sleep duration and reductions in anxious/depressed behavior, withdrawn behavior, and aggressive behavior. Even after adjusting for the other confounding variables (e.g., poverty and low levels of maternal education), longer sleep duration is associated with better behavioral outcomes in this population. Thus, given the strong association that we found between language-based bedtime routines and sleep duration, coupled with a growing literature on the association between short sleep duration and children being overweight and obese (Chen, et al., 2008
; Marshall, Glozier, & Grunstein, 2008
; Patel & Hu, 2008
), we were surprised to find no association between language-based bedtime routines and obesity. It is possible that a longer time frame than 2 years may be necessary to observe higher levels of overweight and obesity in the children. For example, Snell et al. (2007)
and others (Touchette, et al., 2009
) found associations between short sleep duration and child weight gains over a 5-year period.
Our analyses are limited by potential reporting or measurement error with regard to our measures of sleep duration and bedtime routines. However, prior work suggests reports of both by parents are relatively in line with observations of family behaviors. For example, a similar, parental reported measure of children's bedtime routines, the Bedtime Routines Questionnaire, has exhibited strong internal consistency (α=.88) and moderate correspondence with other measures of sleep quality, sleep hygiene, and routines (r
=.36 to r
=.51) (Henderson & Jordan, 2010
). A more general assessment of children's daily routines, the Child Routine Inventory, has also shown strong internal validity (α=.90) and test-retest reliability (r=.86) (Sytsma, Kelley, & Wymer, 2001
). Furthermore, previous research suggests parental-reports of sleep patterns in young children have adequate reliability and validity (LeBourgeois, 2003
; Sadeh, et al., 2003
). Other limitations include lack of information on daytime napping behaviors and other sleep behaviors, as well as more details about the characteristics of the language-based bedtime routine (e.g., length of activity, regularity, timing, and who carried out the activity). In addition, we had smaller sample sizes for the outcome measures that required in-person measurement (i.e., PPVT and obesity) compared to those that could be collected by phone (i.e., behavior and health status) Finally, while bi-directional associations may exist between verbal-related routines, sleep duration, and health/developmental outcomes, our longitudinal design provides robust temporal assessments suggesting bedtime activities of a verbal nature at an early age best predict later language outcomes. Nonetheless, it is important to consider that bedtime routines are likely to be serially associated. Indeed, although FFCW did not collect data on specific types of routines at the age-5 assessment, families were asked whether they engaged in any bedtime routines; 81 percent of those families who engaged in one or more bedtime routines at age 3 also engaged in bedtime routines at age 5.
Overall, our results indicate that language-based bedtime routines are associated with both increased sleep hours and improved cognitive skills. As is the case with all observational studies, our estimates do not reflect causal relations, despite that these findings withstand substantial adjustment for potentially confounding variables. Future research should seek to further investigate these findings, perhaps in the context of a language-based bedtime-routine related intervention study. In addition, future research should seek to understand the role of maternal emotional availability in moderating or mediating positive associations of language-based bedtime routines with preschoolers' sleep and cognitive outcomes. Finally, given evidence of similar socioeconomic risk factors with regard to both sleep-related problems and poor developmental outcomes for children, educational campaigns around sleep and sleep-related behaviors—particularly those that encourage language-based bedtime routines—may want to target low socioeconomic households.