People and other animals sicken and die if they are deprived of sleep, but why is sleep so essential? Psychiatrists proposed the ‘synaptic homeostasis hypothesis’ (SHY) in 2003. This hypothesis holds that sleep is the price we pay for brains that are plastic and able to keep learning new things. A few years ago, they started research that could show direct evidence for their theory. The result offers visual proof of SHY.