The activation of sleep regulatory processes by an extension of wakefulness from 16 to 40 h results in a relative increase of low frequency EEG activity that is most pronounced in frontal cortical areas. verified on the day of admission to the laboratory with urinary toxicologic analysis. Subjects were instructed to keep a regular sleep-wake schedule (bedtimes and waketimes within±30 minutes of self-selected target times) during the three weeks prior to their admission to the laboratory. Adherence to a regular schedule during the week immediately prior to admission was verified with a wrist actigraph (Mini Motionlogger, AMI, Ardsley, NY, USA). All subjects gave their written informed consent. The protocol, consent form and advertisements were approved by the Human Research Committee of the Brigham and Women's Hospital.