Acute effects of light on body temperature and activity in Syrian hamsters: influence of circadian phase

Light exposure at night causes an acute increase in human body temperature, which normally falls during the night. This change is largely attributable to the suppression by light of the nocturnal rise in melatonin levels. Lit tle is known, however about the effects of light on body temperature in noc turnally active mammals in which the nightly peak in melatonin secretion co incides with the circadian phase of elevated, rather than decreased, body t emperature. We investigated the effects of a l-h exposure to light on body temperature and activity of Syrian hamsters, Mesocricetus auratus, at two p hases during the night and at two phases during the projected day. Brain or abdominal temperature was recorded continuously using implanted radio tran smitters while locomotor activity was monitored simultaneously using a pass ive infrared movement detector. Responses to light exposure were strongly c ircadian phase dependent; light during the night caused elevations in both brain and core body temperature, whereas light; during the projected day di d not. Temperature increases at night could not be attributed solely to act ivity increases at the onset of light pulses, indicating a contribution fro m nonbehavioral mechanisms of thermogenesis. These results provide the firs t evidence for circadian modulation of acute temperature responses to light in a nocturnal mammal.