EFFECTS OF TWILIGHTS ON CIRCADIAN ENTRAINMENT PATTERNS AND REENTRAINMENT RATES IN SQUIRREL-MONKEYS

Entrainment patterns of the circadian rhythms of body temperature and locomotor activity were compared in 6 squirrel monkeys (Saimiri sciure us) exposed to daily illumination cycles with abrupt transitions betwe en light and darkness (LD-rectangular) or with gradual dawn and dusk t ransitions simulating natural twilights at the equator (LD-twilight). Daytime light intensity was 500 lux, and the total amount of light emi tted per day was the same in the two conditions: Mean daytime body tem perature levels were stable in LD-rectangular but increased gradually in LD-twilight, reaching peak levels during the dusk twilight. Locomot or activity showed a similar pattern, but with an additional, secondar y peak near the end of dawn. Activity duration was about 0.5 h longer in LD-twilight than in LD-rectangular, but the time of activity midpoi nt was similar in the two conditions. Reentrainment of the body temper ature rhythm was faster following an 8-h advance of the LD cycle than following an 8-h delay, but did not differ significantly between the t wo LD conditions. These results provide no evidence that the inclusion of twilight transitions affected the strength of the LD zeitgeber, an d suggest that the observed differences in the daily patterns reflecte d direct effects of light intensity on locomotor activity and body tem perature rather than an effect of twilights on circadian entrainment m echanisms.