Circadian regulation of sleep in larvae. Circadian and homeostatic (sleep-dependent) regulation of circuit-dependent rhythmic structural synaptic plasticity. (A) Zebrafish larvae were kept under LD for 6 days. At 6-8 dpf, sleep was monitored under constant dim light for three consecutive days (gray and black bar represent subjective day and night, respectively). Sleep was defined and monitored as previously described (Elbaz et al., 2012). Sleep time was rhythmic and peaked during the night (n = 55). (B,C) A proposed model demonstrating circadian and sleep/wake regulation of structural synaptic plasticity in the brain. Rhythmicity of synapse number, size and location is affected by: (B) the circadian clock (C) homeostatic process (sleep and wake). (B) While the number of synapses in a given circuit I increase during the day, the circadian clock could drive, at the same time, a reduction in synapse number in circuit II. (C) In parallel, homeostatic process controls the number of synapses in both circuits I and II. These two processes may be opposed or additive. Thus, the identity and role of a specific circuit determines its relative regulation by the circadian and homeostatic processes. (D) Imaging of synaptic fluorescence marker in live zebrafish larvae. This technique enables monitoring of structural synaptic plasticity in specific circuit during day and night, sleep and wakefulness.