ABSTRACT
It has been recognized since the first studies of humans in time-free environments that the timing of major nighttime sleep is governed in large part by the endo genous circadian pacemaker (1,2). The dependence of sleep on the biological clock is reflected in the characteristic temporal relationship between sleep propen sity and the circadian rhythm of body core temperature. When subjects are studied in environments free of time cues, the onset of major sleep episodes tends to occur in close proximity to the trough of body temperature (3,4).
