The gut microbiota and sleep in infants: a focus on diurnal rhythmicity patterns

Emerging evidence supports a bidirectional relationship between the gut microbiome and sleep, partly mediated by the microbiota-gut-brain axis. Circadian rhythms appear to influence early developmental processes and may play a role in shaping this relationship. While infancy is a critical window for both gut microbiota establishment and the maturation of sleep regulation, the role of gut bacteria in modulating infant sleep remains poorly understood. In this longitudinal study, we performed continuous within-subject sampling (monitoring across 48 hours) in 20 infants at 2, 4, and 6 months of age to examine associations between the establishment of gut microbiota and sleep patterns. We hypothesized that microbial diversity and sleep rhythmicity would be linked across both short (diurnal) and long (monthly) time scales. Gut microbiota profiles were characterized using 16S rRNA gene sequencing; gut melatonin concentrations were measured; and sleep metrics were quantified through wearable actimetry, parent-reported 24-hour sleep diaries, and the Brief Infant Sleep Questionnaire (BISQ). Parents additionally completed the Baby Care Questionnaire (BCQ), assessing parenting style, and the Ages and Stages Questionnaire (ASQ), evaluating infant behavioral development. In some infants, alpha diversity followed diurnal rhythmic patterns. Although bacterial rhythmicity was not significantly associated with sleep rhythmicity, as quantified with the Circadian Function Index, infants with higher alpha diversity had more robust sleep rhythmicity ({beta}Faith PD= 0.0721, SE = 0.0337). Infant age emerged as the strongest predictor of both gut microbiota diversity (observed features: {beta}age = 0.101, SE = 0.0172) and gut melatonin concentrations ({beta}age = 0.638, SE = 0.162). For the same age, gut microbiota temporal volatility - an indicator for bacterial community instability - was not associated with sleep patterns. This study demonstrates promising short- and long-term links between gut microbiota diversity and sleep-wake rhythm maturation in infancy. Further research is needed to elucidate the mechanistic role of the gut microbiome in sleep development.