Circadian and homeostatic influences on sleep in the squirrel monkey: Sleep after sleep deprivation

A series of sleep deprivation (SD) experiments were performed to examine th e relative influence of circadian and homeostatic factors on the timing of sleep in squirrel monkeys free-running in constant illumination. All SDs st arted at the beginning of subjective night and lasted 0, 1/4, 1/2, 1, 1 1/4 , or 1 1/2 circadian cycles. These six lengths represented three pairs: (0, 1), (1/4, 1 1/4),(1/2, 1 1/2). Within each pair, SD ended at the same circa dian phase but differed by one circadian cycle in duration. Both before and after SD, consolidated sleep (CS) episodes occurred predominantly during s ubjective night, even after long SDs ending at the beginning of subjective day. CS duration,was strongly influenced by circadian phase but had no over all correlation with prior wake duration. Sleep loss incurred during SDs lo nger than 1/4 cycle was only partially recovered over the next two circadia n cycles, though total sleep duration was closer to baseline levels after t he second circadian cycle after SD. There was a trend toward a positive cor relation between prior wake duration and the amount of NREM and delta activ ity measures during subjective day. Delta activity was not increased in the first 2 hours of CS after the SD. Relatively high levels of delta activity occurred immediately after the SD ended and again at the time of baseline CS onset. These data indicate that the amount of sleep and delta activity a fter SD in squirrel monkeys is weakly dependent an prior wake duration. Cir cadian factors appear to dominate homeostatic processes in determining the timing, duration and content of sleep in these diurnal primates.