Effect of melatonin on sleep and brain temperature in the Djungarian hamster and the rat

The effect of a single dose of melatonin (3-5 mg/kg intraperitoneally) on sleep, electroencephalographic power density, and cortical temperature (TCRT) was investigated. Melatonin was administered to Djungarian hamsters 4 h or 12 h after lights on in a 16-h light:8-h dark cycle (LD 16:8) and to rats at dark onset in a LD 12:12. The effects in both species were short lasting and depended on the time of day. Sleep latency was prolonged in the late light period, sleep fragmentation was enhanced in the early light period, and TCRT was elevated in all three conditions. Rapid eye-movement sleep was reduced in the first postdrug hour after the late light period treatment in the hamsters and in postdrug hours 2 and 3 after dark onset treatment in the rat. Therefore, we have no evidence for a sleep inducing effect of 3-5 mg/kg of melatonin in the hamster or rat. In view of some data that indicate that melatonin may exert a sleep inducing effect in humans, it is suggested that melatonin induces changes that are typical for the dark period of each species, i.e., waking in the nocturnal Djungarian hamster and rat, and sleepiness in the diurnal human.