Removal of temporal constraints in the middle-aged and elderly: effects on sleep and sleepiness

Ten healthy middle-aged and elderly subjects (44-81 years old; 4 men and 6 women) were studied in time isolation experiments involving: (a) entrainment to the subject's habitual routine (EN condition) and (b) removal of all temporal constraints or "free-running" (FR condition) with the subjects able to choose their own bed and meal times. Subjective day lengths and circadian temperature cycle periods were longer under FR than under EN, but not by an equal amount. Perhaps as a consequence, although time-in-bed (TIB) fractions were greater under FR than under EN, actual sleep fractions were not, reflecting a decline in sleep efficiency. Neither was there an increase in slow-wave sleep (SWS) under FR compared with EN. Daytime subjective alertness did, however, increase upon release into FR, also showing a slightly earlier peak than that observed under the EN condition. The results point to the need to regard sleep and sleepiness as rhythmic, as well as homeostatic, processes.