phylogenetic evolution of sleep in animals

Sleep in mammals is quite precisely regulated, as a constant daily quota is maintained through a balance between its duration and intensity, a phenomenon referred to as sleep homeostasis (Landolt, 2008). One of the measures of sleep intensity is related to the number of delta brain waves observed in the electroencephalogram during the slow sleep phase or NMOR (without rapid eye movements). Other physiological variables, such as arousal threshold, sleep continuity, motor activity, heart rate and respiratory activity, are also useful indicators to identify sleep depth.