L5 onset as a proxy for circadian phase in infants

Chronotype is commonly assessed using the mid-sleep point as an indicator of circadian phase. In infants, however, this assessment typically relies on actigraphy, and most available sleep-scoring algorithms have been developed and validated for adolescents and adults, limiting their applicability in early childhood. The aim of this study was to examine the relationship between the mid-sleep point and L5 onset, a nonparametric measure derived from rest–activity rhythms that reflects the start of the least active 5-h period. A total of 502 nights from 81 six-month-old infants were analyzed. Sleep onset and offset were determined by visual inspection of actograms, and the mid-sleep point was subsequently calculated. A positive correlation was observed between mid-sleep point and L5 onset (r = 0.22, p < 0.001). Linear mixed-effects models indicated significant associations both between infants (β = 0.63, p < 0.001) and within infants (β = 0.10, p < 0.001). These findings suggest that L5 onset may serve as a practical proxy for circadian phase in 6-month-old infants, offering an alternative to sleep parameters derived from algorithms not specifically validated for this age group.