Burning the midnight oil? Pulling an all-nighter? There’s more than one way to describe staying up late, and there’s also more than one consequence. Trouble staying focused is one of the more immediate effects of shorting your brain on sleep — and a new study from MIT suggests why. When you clock out and fall asleep, your brain’s cleaning crew clocks in. Deep, restorative sleep is when your neurological housekeeping occurs. Your brain is swaddled by a pool of cerebrospinal fluid, and during sleep, waves of this fluid mop your proverbial floors to flush out built-up waste from your waking hours. A 2019 study found this “mopping” moved the fluid in a rhythmic pattern, associated with different shifts in people’s brain waves. In the study, participants engaged in two attention tests: once well-rested, and once after disrupted sleep. They pressed buttons in response to a shape changing on a screen, and again when they heard a specific sound. By monitoring the participants’ brain electrical activity, researchers were able to map what was going on when they missed cues or lagged. They noticed the cerebrospinal fluid moving away from the brain when participant attention lapsed, leading the team to believe the brain is trying to “clean” itself during those foggy moments. There were other physical changes too: Heart rates slowed along with breathing and pupils constricted. That suggests this “cleaning” is far-reaching, and points to “a unified circuit” between attention and basic body functions. So, if you find yourself spacing during the day after a night of poor sleep, consider letting your brain catch a break by catching some Z’s. Your ability to focus deserves it.