Figure 1.

Illustration of the laboratory experimental protocol. Participants entered the laboratory on 2 occasions for a duration of 5 days: i.e. during the MF (scheduled on days 5-9 after menses onset) and the ML (scheduled on days 19-23 after menses onset) phases of their menstrual cycle. Each visit was scheduled on a separate menstrual cycle to allow recuperation, and the phase at first visit was balanced such that 4 participants had their initial visit at MF and 4 at ML. After entering the lab in the evening of Day 1 for a period of acclimatization (light levels: 150 lux), participants were put to bed for an 8-h nocturnal sleep episode (light levels: < 0.03 lux) scheduled at their habitual time. Upon awakening, participants began the 72-h URSW procedure which consisted of 36 cycles of 60-min wake episodes in constant conditions (small isocaloric snacks at each wake episode; light levels: < 10 lux) alternating with 60-min nap episodes (light levels: < 0.03 lux), all in semi-recumbent posture. This was concluded with an ad libitum nap episode (light levels: < 0.03 lux). The protocol for a participant with a habitual sleep time from 00:00 to 8:00 is illustrated.