In Wistar rats synchronized to a 12-h light/12-h dark cycle (lights on from 08:00 to 20:00 h), the ip injection of methyleugenol (200 mg/kg) at 08:00, 12:00 and 16:00 h significantly increased the sleeping time (time between loss and recovery of righting reflex) when compared to animals anesthetized at 20:00, 24:00 and 04:00 h. These data provide another example of the importance of circadian rhythms in biological systems and their practical relevance to pharmacology.