Chronic administration of melatonin reduces REM sleep in the Djungarian hamster (Phodopus sungorus).

The prominent effects of photoperiod on sleep, electroencephalogram power spectra and cortical temperature in the Djungarian hamster may be mediated by melatonin. We investigated the effects of chronic subcutaneous melatonin application on these variables in Djungarian hamsters recorded in a short photoperiod (light/dark 8:16 h). Melatonin abolished the light/dark difference in vigilance state distribution, reduced the amount of rapid eye movement (REM) sleep in the light period and the occurrence of REM sleep episodes. In contrast to the reduction of both cortical temperature and electroencephalogram power density in hamsters adapted to a short photoperiod these variables were unaffected by melatonin. Since the effects of chronic application of melatonin differ from those of shortening of the photoperiod it is improbable that melatonin mediates the effects.