Sleep, circadian rhythms, and brain function in fish

This review synthesizes the neurobiological basis of sleep-like states and circadian rhythm mechanisms observed in teleost fish. Fish exhibit a complex biological organisation in terms of sleep and circadian rhythms, neuroendocrine regulation, behavioural adaptation, and interaction with environmental stimuli. Sleep is expressed in many fish species as sleep-like states characterised by behavioural indicators such as decreased motor activity, increased response threshold to external stimuli, and rapid reversibility. This process is regulated by both homeostatic sleep pressure and the circadian clock, which is sensitive to the light-dark cycle. Sleep deprivation in fish negatively affects cognitive performance by causing impairments in learning, memory consolidation, and synaptic plasticity processes. Furthermore, differences in sleep duration and responses to sleep homeostasis across species suggest that sleep physiology has been shaped by ecological and evolutionary adaptations. Thus, fish provide a powerful platform for elucidating the molecular architecture of circadian regulation and for exploring how environmental stressors influence neural function and behavior.