Presynaptic Release Probability Determines the Need for Sleep

Sleep is universal among animals with synapses, yet the synaptic functions determining the need for sleep remain elusive. By directly measuring synaptic transmission at anatomically defined synapses in Drosophila , we found that synaptic strength remained stable or declined after sleep deprivation in a circuit-specific manner. In contrast, presynaptic release probability (P _r ) consistently decreased with sleep loss across circuits and species, stemming from reduced Ca ²⁺ influx or weakened vesicle–channel coupling at presynaptic terminals, and recovered after sleep. Bidirectional manipulations of P _r altered sleep pressure, establishing a causal relationship between presynaptic function and sleep need. Non-synaptic sleep-regulatory signaling pathways consistently modulate P _r but not synaptic strength. Thus, our findings identify P _r , rather than synaptic strength, as the conserved synaptic substrate underlying sleep need.