Four healthy male subjects each experienced a temporal isolation experiment lasting several months. During part of each experiment (2-5 weeks), the subject's entire imposed daily routine (including light-dark, rest-activity, and meal routines) was either stretched (two subjects: T = 25.8 hr, 26.0 hr) or reduced (two subjects: T = 22.8 hr, 23.1 hr) to beyond the range of entrainment of the endogenous circadian pacemaker (ECP), which then ran at a different period (tau). Subjective alertness was measured approximately three times per hour (during wakefulness), using a computerized visual analogue scale technique. Circadian rhythms in subjective alertness were then plotted both at tau, the period length of the ECP, and at T, the period length of the imposed sleep-wake cycle (SWC) and light-dark cycle. At tau, the alertness rhythm was closely in phase with the temperature rhythm. At T, the alertness rhythm showed an "inverted-U" function with a peak toward the middle of the subjective day, upon which was superimposed a "postlunch dip" for one subject. Thus, subjective alertness would appear to be under the control fo both ECP and SWC mechanisms, which combine to produce the composite time-of-day function normally observed in a diurnal setting.