Let Sleeping Zebrafish Lie: A New Model for Sleep Studies

Although the function of sleep is hotly debated, one thing is clear—we, and most other animals, cannot do without it. In a new study, Yokogawa et al. describe how zebrafi sh sleep, fi nding both striking similarities to mammalian sleep and its regulation and intriguing differences. How can you tell when a zebrafi sh is asleep? According to Yokogawa et al., it stops swimming (for at least six seconds), stays immobile at the bottom or on the surface, and becomes less sensitive to external stimuli, such as a mild electric shock. This raised threshold for stimulation is also a feature of mammalian sleep (as is prolonged inactivity). Zebrafi sh, like humans, are markedly diurnal—they sleep more during the night than the day. By using electrical stimulation to prevent the fi sh from sleeping, the authors found another similarity between mammalian and zebrafi sh sleep. Just like mammals, the sleep-deprived zebrafi sh showed a rebound effect: after being deprived of sleep for a time, they slept more, showing that their sleep is homeostatically regulated. But when it came to the effects of light on zebrafi sh sleep, the authors discovered a marked difference from mammalian sleep. When kept in constant light conditions, zebrafi sh barely slept at all. Light could suppress sleep in zebrafi sh almost completely, even if they had been sleep deprived. Surprisingly, the suppression of sleep with light did not produce a rebound effect. When zebrafi sh that had been kept in constant light for three days were returned to darkness, they slept normally, showing no compensatory increase in sleep. The lack of a compensatory rebound effect may more closely resemble sleep in certain birds, such as pigeons, than in mammals. The authors speculate that this strong effect might result from the fact that all zebrafi sh cells are directly sensitive to light and that light suppresses the production of melatonin, a sleep-promoting hormone that is particularly effective in zebrafi sh. Light might act through various pathways to suppress sleep, and this could combine with a lack of melatonin to cause the striking effects of light on sleep in zebrafi sh and to overcome weaker circadian (cyclical) or homeostatic infl uences on sleep. By contrast, in mammals, circadian rhythms and homeostatic drives have a much stronger effect than light does. As vertebrates, zebrafi sh have a nervous system that is similar in overall architecture to our own. Zebrafi …