Sleep in animals

Sleep in animals refers to a behavioral and physiological state characterized by altered consciousness, reduced responsiveness to external stimuli, and homeostatic regulation observed in various animals.^{[citation needed]} Sleep has been observed in mammals, birds, reptiles, amphibians, and some fish,^{[citation needed]} and, in some form, in insects and even in simpler animals such as nematodes.^{[citation needed]} The internal circadian clock promotes sleep at night for diurnal organisms (such as humans) and in the day for nocturnal organisms (such as rats). Sleep patterns vary widely among species. It appears to be a requirement for all mammals and most other animals.^{[citation needed]}