Sleep

Sleep is the regular state of natural unconsciousness observed in all mammals and most fish, reptiles, amphibians, insects, and birds. Sleep is heavily influenced by the Circadian rhythm, a cyclical pattern which occurs daily. The mammalian body clock is based on a 24.5-25.5 hour cycle, and is mediated by the suprachiasmatic nucleus as well as external influencesBefore advances in the fields of neurology, neuroscience, electronics and genetics were made, scientists studied the behavioral characteristics of sleep, such as its pattern, depth and varying frequency. In more recent times, the electrical impulses generated by the brain are recorded using a device called an electroencephalograph (EEG), and individual genes relating to sleep-related brain function, such as the circadian rhythm, are isolated. Molecular biology, medical science and epidemiology all play an important role in modern studies of sleep.
Sleep is often defined using specific criteria relating to EEG data. All mammals and birds fulfill the criteria for sleep based on EEG recordings. In animals where EEG data is not readily available, or their small size precludes recording EEG, behavioral and gene specific data are utilized for sleep studies.