What exactly is your brain doing while you sleep?

EACH OF US CARTS AROUND A 3-LB. UNIVERSE THAT orchestrates everything we do: directing our conscious actions of moving, thinking and sensing, while also managing body functions we take for granted, like breathing, keeping our hearts beating and digesting our food. It makes sense that such a bustling world of activity would need rest. Which is what, for decades, doctors thought sleep was all about. Slumber was when all the intricate connections and signals involved in the business of shuttling critical brain chemicals around went off duty, taking time to recharge. We’re all familiar with this restorative role of sleep for the brain—pulling an all-nighter or staying awake during a red-eye flight can not only change our mood, but also affect our ability to think clearly until, at some point, it practically shuts down on its own. When we don’t get enough sleep, we’re simply not ourselves.

Yet exactly what goes on in the sleeping brain has been a biological black box. Do neurons stop functioning altogether, putting up the cellular equivalent of a DO NOT DISTURB sign? And what if a sleeping brain is not just taking some well-deserved time off but also using the downtime