Humans are naturally diurnal. Their sleep is composed of alternating bouts of Non-Rapid-Eye-Movement Sleep (NREMS) and Rapid-Eye-Movement Sleep (REMS). Their NREMS is subdivided in 3 stages (N1, N2, N3). Their sleep is more continuous than rodents. Rodents are nocturnal animals. Their sleep is composed of bouts of NREMS, REMS and awakenings. Their sleep is more fragmented, bouts are shorter with numerous awakenings.