During sleep, slow pulses of cerebrospinal fluid move through the brain, a nightly tide that helps clear what builds up during the day. Recent research at MIT found that when we go without sleep, these pulses break into waking hours: fluid flows out of the brain as attention lapses, and flows back as attention returns. Awake, thought itself seems to be organized by waves. Rhythms of electrical activity sweep across the cortex, gathering groups of neurons for a task and then releasing them. The tide is both a metaphor for the mind and part of how the brain works.