Why Do We Need to Sleep? Synaptic Homeostasis and the Need for Sleep

Despite the fact that the average person will spend nearly a third of their life asleep, the fundamental function of sleep remains mysterious. Sleep need increases over periods of wake and decreases during ensuing sleep,... [ view full abstract ]

Despite the fact that the average person will spend nearly a third of their life asleep, the fundamental function of sleep remains mysterious. Sleep need increases over periods of wake and decreases during ensuing sleep, consistent with sleep serving a restorative function. The synaptic homeostasis hypothesis proposes that sleep function is tied to learning and memory. Specifically, sleep is proposed to maintain appropriate synaptic activity by globally reducing synaptic strength following changes associated with plastic processes (such as learning and memory) during wake. Consistent with this hypothesis, many studies have demonstrated a correlation between changes in synaptic strength and changes in sleep need. However, a direct causal relationship has yet to be established. We investigated the effects of a pharmacological reduction in synaptic strength on sleep need following an injection of zeta-inhibitory peptide, which has been shown to reduce synaptic strength, into a single cortical area of the rat brain. We found that reductions in synaptic strength produced a near 30% decrease in sleep need of neurons near the injection without affecting sleep need of neurons elsewhere. This suggests that changes in cortical synaptic strength directly regulate the local need for sleep and, more broadly, that sleep may be necessary for continued learning and memory.