Sleep is universal, tightly regulated, and many cognitive functions are impaired if we do not sleep. But why? Why do our brains need to disconnect from the environment for hours every day? One compelling answer to these questions is provided by the synaptic homeostasis hypothesis (SHY), which proposes that sleep is the price the brain pays for plasticity, to consolidate what we have already learned, and to be ready to learn new things the next day. In this talk, SHY will be discussed along with evidence from new experiments showing that the net strength of synapses increases with wake and decreases with sleep. These findings can explain why sleep is necessary for the well-being of neural cells and brain circuits, and how the regulation of synaptic strength may be a universal, essential function of sleep.