Orexin (ORX) A and B (hypocretins) are excitatory neuropeptides produced by neurons of the lateral hypothalamus that have been implicated in the regulation of the sleep-wake cycle. In rats, Fos (the product of the cfos gene) expression shows daily rhythms in areas involved in sleep and wakefulness and orexinergic neurons show elevated Fos expression during the night. The present study directly compared the daily pattern of Fos expression in orexinergic neurons in diurnal (A. niloticus; grass rats) and nocturnal (R. norvegicus; lab rats) rodents. Animals kept on a 12:12 light-dark cycle were perfused at six different Zeitgeber times (ZT), with lights on at ZT 0: 1, 5, 13, 17, 20 and 23. In both nocturnal and diurnal rodents orexinergic neurons showed rhythms in Fos expression, with more Fos seen during the active phase of each species. In the diurnal species, Fos expression in cells of the lateral hypothalamus that do not produce ORX was elevated at ZT 20, a time when these animals sleep, and was low at ZT 13, a time of peak of activity. These results provide further evidence for a link between activity in orexinergic neurons and wakefulness and that in grass rats, other neurons of the lateral hypothalamus may work in an antagonistic fashion with respect to orexinergic neurons to regulate wakefulness in this diurnal species.