What Happens When We’re Sleeping?

When we wake up after a good night’s sleep, our bodies and minds feel refreshed. But a healthy night’s rest is about much more than just feeling rested. During slumber, the body and brain go through critical repair processes that help fight illness, heal wounds, and deal with the stresses of everyday life. And getting adequate amounts of restful sleep has been convincingly tied to better health, a stronger immune system, and a longer life.

The body’s natural rhythm of waking and sleeping has evolved over millions of years, and it is one of the fundamental human needs finely tuned to enable almost all of our body’s functions. But in modern times, our busy lives and a range of mental and physical problems interfere with that cycle, often leading to chronic lack of sleep. In fact, it’s been estimated that 50 to 70 million people in the United States suffer from sleep disorders.

This is partly because of the way we live today: electricity, computers, and smartphones; shift work and travel across time zones; and a culture geared toward daytime activity and not nocturnal rest. But even when adjusting for these factors, getting enough sleep remains a challenge for many adults. And for kids and teens, insufficient sleep can have a serious impact on growth and development.

Researchers are discovering more and more about what happens when we’re asleep. What we’re learning, notably from the use of brain-imaging technology, is that sleep is not the passive state that most people once believed. It is an active process that involves distinct cycles of deep sleep, light sleep, and REM (rapid eye movement) sleep, when we dream.

During the first stage, known as non-REM or NREM sleep, the brain waves slow down and the heart rate and muscles relax. Then, during the next phase, called REM or rapid eye movement sleep, the brain becomes more active and dreams occur. Finally, during the final stage, known as SWS or slow wave sleep, the heart rate and blood pressure rise slightly.

The brain’s glymphatic system clears waste during sleep, removing toxic byproducts of the nervous system that accumulate throughout the day. This helps the brain to function well and support emotional stability. Sleep also contributes to memory, converting short-term memories into long-term ones and erasing those that are no longer needed.

During slumber, the body produces hormones that affect everything from our appetites to stress levels. Not getting enough sleep can lead to a variety of health problems, from obesity and high blood pressure to depression and diabetes. In fact, not getting enough sleep can actually cause some of the same problems as a lack of exercise: it makes your body less sensitive to insulin, which regulates blood sugar. That’s why it’s important to get a full night’s sleep every night.