Overview of the experimental design.

<p>Subjects came to the lab 7 h before scheduled sleep time and stayed for 2 consecutive nights monitored via polysomnography (black box). They stayed under controlled light (<10 lux for wake periods and ≈0 lux for sleep periods) conditions and body posture (dashed lines). During wakefulness, sleepiness and vigilance measures were collected at hourly intervals as well as saliva samples (for melatonin assay). One and a half (morning session) and 10.5 (evening session) hours after scheduled wake up time subjects underwent an fMRI session while performing a Stroop task. Order of morning and evening sessions was counterbalanced between groups and subjects.