Why Can Flies Catch Up Sleep Lost on Weekdays, While We Can’t?

In daytime workers/learners, the reduction of sleep duration on weekdays can be partially compensated by an advance of circadian sleep phase caused by an advance of the 24-h cycle of exposure to light. Since health effects of these sleep reduction and phase advance cannot be easily separated in human studies, Drosophila melanogaster was used to compare health consequences of 1) shaking or heating between 3:00 to 7:00 during 5 weekdays applied to produce the 4-h sleep deprivation on weekday mornings and 2) shifting the 12-h interval of light exposure applied to produce the 4-h advance in weekday circadian timing. Flies showed a gradual restoration of the circadian phase on weekends after the 4-h weekday advance of the 24-h light-dark cycle. However, their reproduction and survival remained uncompromised. Shaking was associated with drastic negative consequences for reproduction and survival. Only survival was compromised by heating. Despite this, sleep loss caused by morning shaking or heating was compensated by its rebound during several following hours, and, on weekends, the 24-h sleep-wake pattern was already fully restored. While humans fail homeostatically compensate their monophasic sleep after its loss in the morning hours, polyphasic Drosophila sleep can be fully compensated during the same weekday.