Study procedure
sleepiness
Participants were asked to keep a regular sleep–wake cycle with a nocturnal sleep of 7 to 8 h prior to study begin and during the nights without recordings. They were asked to keep regular work habits during the entire study.Each participant underwent 2 weeks of recordings, each week containing four nights (baseline, intervention, recovery, recovery) with an intermediate break of 11 days as a wash-out phase between both weeks. Sleep recordings were performed in a German Sleep Society (DGSM) board certified sleep lab. During baseline and recovery nights, participants had an undisturbed night with eight hours of sleep (light off: 11:00 pm, light on: 07:00 am). During the intervention night with sleep fragmentation, participants also slept for 8 h (11:00 pm until 7:00 am) but were woken up 7 times by turning light on every hour. Here, they filled out the 9-point Karolinska Sleepiness Scale (KSS), a short questionnaire on sleepiness. During the intervention night with sleep restriction, participants’ sleep was reduced to 5 h (11:00 pm until 04:00 am), after which there were woken up and spent additional 3 h awake in bed (see Fig. Study design with the interventions restriction (left side) and fragmentation (right side). A randomized cross-over-design was chosen with one group starting on the left side and one group starting on the right side. Light-on and light-off times are shown.