A new study shows that simple early-life habits, like giving yogurt at age one and ensuring steady night sleep, may influence how strongly children develop memory skills by preschool age.
Study: Association Between Working Memory at Age 4 Years and Night Sleep Duration and Yogurt Intake Frequency at Age 1 Year. Image credit: Dragana Gordic/Shutterstock.com
A recent study in Nutrients investigated the effects of sleep duration and yogurt intake frequency on the working memory (WM) of 4-year-old children.
Sleep duration and cognitive development in childhood
Executive functions, a set of cognitive functions that develops during childhood, are essential for higher intellectual activity. This may serve as a determining factor in an adult's capacity to perform complex cognitive tasks. Among preschool-aged children, WM is considered the earliest developing component of executive function. Previous studies have shown that WM starts developing in infancy and improves throughout preschool age, which enables children to retain more information.
Sleep is an essential activity supporting brain structure and function development. It is also associated with executive function performance, so that disturbed sleep may cause impaired executive function. Interestingly, napping was found to promote explicit memory consolidation.
Although some studies have shown that a longer night sleep duration may improve WM performance, others have contradicted this observation. These studies have also indicated that napping does not impact WM performance. Several studies have also highlighted that WM performance is lower in children with shorter night and longer day sleep durations.
Many factors may affect sleep in infancy, including consuming yogurt, a fermented food. Yogurt is a source of probiotics, which positively influence gut microbiota composition, thereby affecting sleep patterns and cognitive functions through the gut–brain axis. However, while previous research suggested yoghurt might affect sleep duration, the present study did not find evidence of such an association. Not many studies have examined the effect of yogurt intake on sleep duration in children, which may influence their WM performance.
About the study
The current study assessed the effect of day, night, and total sleep duration at age 1 (T1), 1.5 (T2), and 3 (T3) years, respectively, on WM at age 4 (T4). The researchers also tested whether yoghurt intake frequency at T1 was linked to sleep duration.
A total of 164 mother-child pairs from the Kyoto Regional Centre of the Japan Environment and Children’s Study (JECS) cohort study were considered. The Sub-Cohort Study included 165 participants aged four who completed the Kyoto Scale of Psychological Development.
Information about annual household income and parents’ education levels was obtained. At T1, mothers completed questionnaires on sleep and yogurt intake. At T2, they completed the sleep questionnaire. In addition to the sleep questionnaire, at T3, mothers completed the Ages and Stages Questionnaire-3 (ASQ-3). At T4, the children performed the digit span task from the Kyoto Scale of Psychological Development (2001) at the Kyoto Regional Centre. Additionally, mothers provided information about their yogurt intake frequency.
Study findings
The digit span scores at 4 years indicated that verbal WM ranged between 1 and 5, averaging 3.40. This score reflected moderate variability in memory capacity. Interestingly, a broader distribution in verbal ability scores was observed among three-year-olds, indicating significant individual differences in language development.
Sleep duration data indicated developmental changes over time. Significant variability in sleep patterns was observed; for example, children at 1 year of age, who slept an average of 12.94 hours per day, exhibited maximum nighttime sleep. Similarly, some children exhibited considerable variability in their daily sleep patterns. As children grew, the total sleep duration gradually decreased, i.e., 1.5-year-olds averaged 12.48 hours, while 3-year-olds averaged 11.43 hours. Day sleep duration also decreased, i.e., 2.62 hours at age 1 to 1.54 hours at age 3.
The current study reflected a wide range of dietary habits among children. In the current cohort, 25% of the children did not consume yogurt, while 28.7% had yogurt 1 or 2 times a week. Approximately 18% and 16% of the cohort consumed yogurt moderately and daily, respectively. Only a very small proportion (about 2%) consumed yogurt more than once per day. These findings imply that although yogurt intake was common among children, high-frequency consumption was relatively rare.
Spearman correlations indicated a significant positive correlation between 1-year-olds’ night sleep duration and 4-year-olds’ verbal WM. This finding suggests that longer night sleep in infancy supports later memory development. However, no significant associations were found between WM and day or total sleep duration at any age. A marginal link between 3-year-olds’ total sleep duration and 4-year-olds’ WM was observed, highlighting a potential developmental link.
Yogurt intake at one year of age exhibited a significant positive correlation with 4-year-olds’ verbal WM. Furthermore, early sleep and nutrition are found to be more closely related to WM development than to verbal ability. Interestingly, annual household income was significantly correlated with yogurt intake frequency only in 1-year-olds. Hierarchical regression analysis indicated that yoghurt intake became significant at the third stage, while night sleep duration at age 1 was only marginally significant at the fourth stage. Verbal ability scores at age 3 significantly contributed to the variance in verbal WM scores at age 4.
Conclusions
This study supported the hypothesis that early dietary habits were associated with later cognitive outcomes. It was noted that one-year-olds’ yogurt intake frequency and night sleep duration each showed positive associations with verbal WM scores at age 4.
However, the observed effects were small, and the association between night sleep and WM was weaker than that of yoghurt intake. The study's findings must be validated in the future based on a larger and more diverse cohort.
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