Some animals never exhibit a state that meets the behavioral definition of sleep. Others suspend or greatly reduce ‘sleep’ behavior for many weeks during the postpartum period or during seasonal migrations without any consequent ‘sleep debt.’ Rats die from one form of sleep deprivation, but sleep loss has not been shown to cause death in well-controlled studies in other vertebrate species. Some marine mammal species do not show evidence for REM sleep, and convincing evidence for this state in reptiles, fish and insects is lacking. The enormous variation in the nature of rest and sleep states across the animal kingdom and within the mammalian class has important implications for understanding the evolution and functions of sleep.
Sleep in fish
There are more than 30 000 species of fish [23]. They vary in size, diet and ecological specialization. Fewer than 10 fish species have been examined for rest or sleep behavior in laboratory studies.
In studies of rest/sleep-like activity of the zebrafish (Danio rerio), circadian variations in responsiveness and activity and decreased response to stimuli were seen after rest deprivation, leading the authors to conclude it was a ‘sleep-like’ state [31,32]. The state characterized as ‘sleep’ could be completely blocked for long periods by light, with no evidence of subsequent rebound [31]. Zebrafish with a null mutation of the receptor for the peptide hypocretin have a substantial decrease in sleep behavior, the opposite of the syndrome seen with loss of this receptor in mammals [33]. The anatomy of systems known to have a major role in sleep–waking control in mammals is radically different in zebrafish [31].
Activation of the perch (Cichlosoma nigrofasciatum) by light during the normally inactive period produced an increase in rest behavior during the subsequent 12 h period [34]. As the authors point out, these effects could be due to the stimulatory effects of light rather than the induced motor activation. No evidence for elevated response threshold during the ‘rest’ state in baseline or rebound conditions was presented and the authors do not claim to have demonstrated the presence of sleep. In another study, activity of a school of Tilapia mossambica was monitored and a smaller response to electrical stimulation was noted during inactive periods. A study of several species of coral reef teleosts reported continuous daytime activity and continuous nighttime activity termed ‘sleep swimming’ [35].
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