All animals sleep and it has long been established that sleep plays a role in the consolidation of memories. Now it looks as though there is another reason to snooze. Lulu Xie of the University of Rochester Medical Center and colleagues used real-time assessment of tetramethylammonium diffusion and two-photon imaging in mice to show that both natural sleep and anaesthesia increase interstitial space by 60%, boosting convective exchange of cerebrospinal fluid with interstitial fluid. This boosts β-amyloid clearance and suggests that sleep is the brain’s way of clearing out potentially toxic metabolic waste products of neural activity. Sleep deprivation not only impairs cognitive function, it kills rodents and flies in days to weeks and causes dementia and eventually death in humans within months to years.