# Zebrafish sleep displays distinct sub-states

## TL;DR

Zebrafish sleep includes distinct light and deep sleep states, similar to mammals, and is regulated by neuromodulators like melatonin, serotonin, and norepinephrine.

## Contribution

The study identifies behaviorally defined sleep sub-states in zebrafish and shows their regulation by specific neuromodulators.

## Key findings

- Zebrafish exhibit two sleep sub-states analogous to mammalian light and deep NREM sleep.
- Sleep deprivation causes a deep sleep-dominated rebound, and neuromodulators selectively alter sleep proportions.
- Melatonin, serotonin, and norepinephrine differentially regulate sleep depth and timing.

## Abstract

Sleep is an essential and evolutionarily conserved behavior. While mammals and several other species have been shown to exhibit well-defined sleep sub-states, it remains unclear to what extent such differentiation exists across the animal kingdom. Here we show, using long-term behavioral data combined with Hidden Markov Modeling, that larval zebrafish display two behaviorally defined sleep-related sub-states: a low-activity state consistent with light sleep and a quiescent state consistent with deep sleep. These are likely analogous to light and deep NREM sleep in mammals. Although both states occur primarily at night, arousal responses differ by sleep sub-state, and sleep deprivation induces deep sleep-dominated rebound. Moreover, the proportions of deep and light sleep are selectively altered by genetic and pharmacological manipulations of melatonin, serotonin, and norepinephrine signaling, offering new insights into how these neuromodulators shape sleep architecture. These results support zebrafish as a tractable model for dissecting the regulation and function of sleep sub-states. More broadly, they demonstrate that structured, multi-state sleep is a conserved feature of vertebrate behavior.

•Hidden Markov models identify deep and light sleep states in freely behaving zebrafish•Light exposure is required to maintain high-activity waking behavior•Melatonin mediates the circadian oscillation of deep sleep•Serotonin and noradrenaline differentially regulate sleep depth

Hidden Markov models identify deep and light sleep states in freely behaving zebrafish

Light exposure is required to maintain high-activity waking behavior

Melatonin mediates the circadian oscillation of deep sleep

Serotonin and noradrenaline differentially regulate sleep depth

Behavioral neuroscience; Neuroscience