Sleep, you have heard, is essential for human health and development. Newborns and toddlers tend to spend more than half of every day asleep. And while many adults try to get by with less, most people need between seven and nine hours each night in order to perform and feel their best.
While there’s no doubt that sleep is necessary, there’s still a lot about it that befuddles sleep experts. “Despite years of scientific research and studies, we still don’t completely understand why we need to sleep,” says says Dr. Leila Kheirandish-Gozal, a sleep research and professor of paediatrics at the University of Missouri School of Medicine. “We also still don’t know why we dream.”
What experts do know is that sleep is a surprisingly active and fertile time for the brain. Sleep seems to play a crucial role in helping your brain sort, process, store, and make use of the stuff you encounter during your waking hours.
“Some people think of it as a waste of time — as turning the brain off,” says Dr. Carl Bazil, director of the Division of Epilepsy and Sleep at the Columbia University College of Physicians and Surgeons. “But we know now there’s a lot of brain activity going on, and all different types of activity depending on the stage of sleep you’re in.”
Bazil says a pile of research has established that sleep helps people turn short-term memories into durable, long-term ones. And sleep also helps your brain sift out and dispose of the jetsam that’s not worth holding onto, more research suggests.
Sleep also appears to assist the brain with meaning-making and other complex aspects of learning. For example, evidence shows that sleep strengthens “relational memory,” which is a kind of logical, inference-based intelligence that allows people to put two and two together in order to make good decisions.
Sleep may also play a role in motor-task learning. Musicians, dancers, and athletes often find that, following a good night’s sleep, they’re much better at a new skill than they were the day before when they quit practicing, Bazil says.
While your brain is able to engage in some of these chores while you’re awake, the hours you spend asleep seem to do all this more thoroughly and efficiently.
“Sleep is an evolutionary function that seems to be extremely important for our health and survival.”
“The overriding theme is that your brain can’t constantly be bombarded with information and be expected to process it,” Bazil says. In many ways, it’s accurate to think of your waking hours as the data-collection phase of your day, while the hours you spend asleep are the time when your brain sorts and makes use of that information. Skimp on sleep, and you handicap your brain’s processing power.
But these cognitive duties are a mere snapshot of sleep’s panoramic range of responsibilities.
“Sleep is an evolutionary function that seems to be extremely important for our health and survival,” says Kheirandish-Gozal.
She says mood is regulated by sleep, and that a lack of quality rest can result in irritability, depression, anxiety, and behavioral changes. There’s also evidence that metabolic waste is cleared out of your brain and body while you sleep, and that sleep promotes cellular repair and recovery.
This waste-removing function may be especially critical when it comes to a person’s risk for disease. Some high-quality research in mice has shown that cerebral spinal fluid, which flows through the brain and removes accumulated proteins and other detritus, all but trickles when mice are awake. But its flow surges during sleep. Protein accumulation is one hallmark of Alzheimer’s disease, and so the protein-clearing function of sleep may play a part in the disease’s development.
In fact, pick a medical condition, and there’s probably research showing that poor sleep makes it worse. A study from the University of Chicago found a lack of sleep can impair the way fat cells respond to insulin, which over time could promote the development of obesity, diabetes, and other metabolic disorders. Poor sleep is also implicated in the progression of heart disease. “We know that correct and healthy sleep causes a dipping of blood pressure, and this dipping helps reduce the risk of heart disease later on,” Kheirandish-Gozal says.
But in many cases, new sleep discoveries have led to more questions than answers. Sleep experts have traditionally broken sleep down into four stages of activity — including REM and non-REM sleep. But research published this year finds that sleep-related brain activity is varied and dynamic, and may be more accurately divided into dozens of stages that each have their own unique utility.
“We know sleep is useful, but when you ask why we do it in the first place — why every animal, including insects and worms, go through it — there’s really no answer to that,” Bazil says. “All we can really say is that it’s important.”
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