To study the temporal organization of daily hypothermia and torpor in a nocturnal Malagasy primate, the gray mouse lemur, body temperature (T(b)) and locomotor activity were recorded using telemetry on 39 males held in 24-h light-dark cycles of different photoperiods. Under free-running condition, the circadian T(b) and locomotor activity rhythms had a period shorter than 24 h. Circadian daily hypothermia started by a rapid drop in T(b) (0.24 degrees C/10 min) at the end of subjective night (13 h 25 +/- 20 min) and was characterized by minimal T(b) values 3 h 20 +/- 5 min later. Spontaneous arousal from daily hypothermia occurred at a fixed time (6 h 05 +/- 15 min, n = 7) after the beginning of subjective day. In animals exposed to 24-h light-dark cycles with night duration varying from 10 to 14 h, locomotor activity was strictly restricted to dark time, but the temporal organization of daily hypothermia was not modified, although changes in amplitude of T(b) rhythm were observed. Daily hypothermia was directly induced by light and lasted 5 h 10 +/- 10 min, with minimal T(b) values 3 h 30 +/- 30 min (n = 28) after lights on, on condition that nighttime did not exceed the duration of subjective night. However, in animals exposed to 24-h light-dark cycles with night duration varying from 10 to 5 h, the limit of induction of daily hypothermia by light was ~9 h after the beginning of night. Finally, under short days (14:10-h light-dark cycle), long bouts (6 h 50 +/- 40 min) of actual torpor (minimum T(b) 27.6 +/- 0.9 degrees C) were observed and would involve mechanisms depending on physiological changes induced by short day exposure.