Circadian sleep-wake cycle organization in squirrel monkeys.

To investigate the relationship between circadian rhythms of body temperature and sleep-wake stages, four squirrel monkeys were prepared for unrestrained monitoring of temperature, locomotor activity, electroencephalogram, electroculogram, and electromyogram. Continuous records for each animal were made for several 12-h light-dark (LD) cycles and then after a few days in constant illumination (LL). All animals maintained consolidated sleep-wake cycles and had a longer circadian period (mean 24.7 h) in LL than in LD (mean 24.1 h). The increased period reflected greater time per circadian cycle spent awake in LL (mean 14.0 h) than in LD (mean 12.8 h). Total night NREM sleep was less in LL (mean 6.5 h) than in LD (mean 8.2 h). Sleep onset occurred at later phases in LL (187 +/- 6 degrees) than in LD (170 +/- 2 degrees). Because the circadian phase measure of NREM sleep was unchanged between LD and LL conditions, the difference in sleep onsets reflected balanced changes in NREM circadian waveforms. Wake-up phases were the same in both conditions (mean 342 degrees). In summary, during free run squirrel monkeys maintain a stable consolidated circadian sleep-wake cycle with a period greater than 24 h, but they exhibit only minimal internal phase restructuring.