We don't know exactly why we sleep. Sleep researchers differ on their conclusions about periods of inactivity in animals and what they represent, whether they equal sleep. ‘The reasons for sleep’ details how various classes of animals ‘sleep’ including mammals, birds, reptiles and amphibians, fish, and invertebrates. There are various theories as to why animals sleep: cellular restoration; energy conservation; and brain efficiency. The debate as to what sleep is has marginalized the fundamental question of why each species seems to have evolved a specific pattern of rest and activity within a 24-hour cycle. The answer is not yet known.