We think of sleep as a passive activity that just makes us feel rested. But sleep is actually hard at work — repairing muscles, cleaning the brain, forming memories, and more. And it’s crucial to your health: Getting enough of it reduces your risk for heart disease, obesity and mental illness. It also helps boost athletic performance and help your immune system fight off infections.

Scientists spend much of their waking hours trying to understand what exactly happens while you’re asleep. The answers can be powerful — and sometimes surprising.

In humans, three criteria separate sleep from other states: a closed eyelid, reduced sensory stimulation (no light, no sound) and a lack of presleep activities that would lead to arousal. However, subjective experience and verbal reports of such experiences are often at variance with both behavioral classifications and sleep electrophysiology. This has led to some controversy about whether certain pre-arousal states are indeed sleep.

The best-known and probably the most important function of sleep is its restorative work on the body, including healing the heart and blood vessels and removing waste products from the cells. Sleep also affects our metabolism and, along with diet and exercise, can reduce the risk of high blood pressure, diabetes and heart disease. But despite its critical role, many people don’t get enough.

When you’re sleep deprived, your body and mind become less responsive, leading to problems like depression, memory loss and difficulty concentrating. A lack of sleep also suppresses the immune system, making you more susceptible to infection. But the good news is that you can make some simple changes to your lifestyle and habits to get more restful sleep.

As you fall into a deep sleep, your heart rate slows, and your body temperature drops. Your brain waves slow down and you experience short bursts of electrical activity called sleep spindles. Research suggests that these bursts may help consolidate memories. During the next cycle, you move into REM sleep, which is more like active wakefulness and you dream. Throughout the night, you’ll go through four or five cycles of non-REM and REM sleep.

Researchers are also learning more about how sleep affects the brain. For example, a study published in 2021 found that being in stage one of sleep for 15 seconds caused a “creative spark,” similar to what you might experience if you’re exposed to something stimulating while awake.

Sleep is a universal human need, but its mechanisms are still under discovery. For example, the reason that sleep seems to help prevent Alzheimer’s disease is still a mystery. Other mysteries include how some individuals have such a strong need for sleep, which is influenced by genetics and their environment, and what triggers the onset of insomnia.

A growing number of studies suggest that sleeping well is key to physical and mental health. A healthy lifestyle, including a balanced diet, regular exercise and social connections, can improve your sleep. Likewise, treating underlying conditions such as anxiety and depression can help you sleep better.