Can the Sun Prevent Weekend Sleep Advance After Early Weekday Wakeups?

There is sparse evidence on how circadian sleep timing is affected by 5 days on/2 days off school/work schedule. In an in silico study, we applied a model of sleep-wake regulation to highlight the difference in predictions based on two alternative explanations proposing either sun time or social time (eg, either solar midday or weekday risetime) as the major contributor to light entrainment of sleep timing. Self-reported sleep times were then used to confirm these predictions. The difference between earlier and later weekday risers in weekend sleep timing their difference in weekday risetime were compared. This difference in weekday risetime is equal to the sum of differences in sleep phase shift on weekends and sleep loss on weekdays measured as the differences in weekend risetime and weekend-weekday gap in risetime, respectively. Three sets of samples were used for these estimations: 87 and 100 paired samples obtained before vs during lockdown and during early vs later school start time (five and three age subsets, respectively) and 1250 vs 1192 unpaired samples with weekday risetime not earlier vs later than 7 a.m. (five age subsets). In any age subset, a shift in social time (weekday risetime) caused a shift in weekend sleep phase (weekend risetime) due to a shift in the 24-h pattern of exposure to light, but this sleep phase shift was less pronounced than the shift in social time. Both social time and sun time substantially contribute to the light entrainment of circadian sleep timing.