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It’s no secret that we’re a nation of sleep deprivation. A study in the journal Sleep found that from 2012 to 2017, the number of Americans who got fewer than six hours of sleep per night increased by roughly the entire population of New York City. This adds up to one third of the total U.S. population, according to the Centers for Disease Control and Prevention, who routinely forgo the recommended seven to nine hours for a full night’s rest.

“Think of sleep deprivation as a 24-hour news cycle, where there is no time to assimilate and digest: Everything is new, and yet everything is forgotten,” Chiara Cirelli, Ph.D., and Giulio Tononi, Ph.D., share with Thrive. Cirelli and Tononi are psychiatrists at University of Wisconsin-Madison and co-authors on a pivotal 2017 study on what goes on in our brain while we’re sleeping.

Turns out that’s a lot: Your brain takes advantage of the quiet time to engage in one of the most underreported, though crucial benefits of sleep: forgetting. Forgetting is actually critical for a healthy brain. In fact, we can’t function without it.

On any given day, we’re inundated with stimuli from our environment, our own bodies, and our social interactions. Our mind interprets these into memories, feelings, actions, and experiences. But in order for our brain to make sense of it all — and to designate what’s actually important — it needs proper downtime: sleep. While we’re asleep, our brain sorts through everything we took in and put out that day, encoding memories into vast networks of neural pathways, patterns of neurons, the primary brain cells, and synapses, effectively neural cables that connect the neurons to each other. These pathways either strengthen or wither over time; each time a memory is recalled, its neural pathway is activated and fortified for the next recall. And memories that are increasingly less important and relevant begin what’s called synaptic pruning: the synapses progressively weaken, giving way for newer neural routes to form and flourish the next day.

Cirelli and Tononi found that our synaptic connections actually shrink by about 20 percent while we’re asleep — and that’s a good thing. It means the brain’s billions of neurons and trillions of synaptic connections are recuperating so they can bounce back afresh in the morning, a restorative process called synaptic renormalization. Cirelli and Tononi also refer to this as “smart forgetting”: “It integrates new memories optimally with old ones, keeping what fits best and getting rid of the rest. It also makes us ready to learn again in the morning, creating space for the acquisition of the new,” Cirelli and Tononi explain. When we skimp on our sleep, these neural pathways remain more like taut rubber bands, stretched to capacity from “information overload.”

This sheds new light on some of our most vexing concerns of mental health. One prevailing psychiatric theory holds that synaptic pruning run amok could manifest as schizophrenia, whereas too little synaptic pruning could manifest as autism spectrum disorder. While there’s no causation to worry about here, the continuum shows just how pivotal a role forgetting plays in our day-to-day cognitive functioning. “In the brain, like in a properly functioning nation, sleep may provide a healthy balance between change and stability,” Cirelli and Tononi say.

If you find it hard to wind down after running through your day at a mile a minute, try cozying up with a warm mug of caffeine-free tea and good read. The soothing heat slightly elevates your core body temperature — and the ensuing drop signals to your brain that it’s time to quiet down and prepare for its restorative night shift.

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