The study of sleep is a relatively new field, but scientists already know that sleep is much more than just a time when your brain shuts down. The body does many important things while you’re asleep, including repairing cells damaged during the day, processing your emotions and preparing for what comes next. It’s no wonder that getting enough sleep is essential to good health.

Insufficient sleep has been linked to a wide range of health problems, from heart disease and stroke to obesity and diabetes. A lack of sleep can even increase the risk of mental illness and mood disorders, like depression and anxiety. Thankfully, there are ways to improve your sleep, and some of the best practices involve changing the way you think about sleep. Instead of seeing it as a waste of time, think of it as one of the most productive things you can do during your lifetime.

There are two key processes that regulate your sleep: circadian rhythms and sleep drive. These are the same systems that control your daily cycles, such as eating and exercise. Throughout the day, your body craves sleep in the same way that it hungers for food. But unlike hunger, where you can’t force yourself to eat, sleep is impossible to resist when your body reaches its sleep drive threshold.

Once you reach your sleep drive threshold, the hypothalamus signals a series of changes that begin to promote sleepiness. Then the nerves that control your muscles reduce their activity, and the electrical impulses that drive most of your brain activity slow down. As these and other changes take effect, the brain releases a chemical called adenosine triphosphate (ATP), which gives your cells energy. During sleep, the brain flushes out waste produced by your neurons, and your memories are pruned.

During stages three and four of sleep, the brain is very active, but less so than during wakefulness. You spend more time in REM sleep during these phases, and this is when you dream. It’s believed that these dreams are important for our emotional well-being.

During REM sleep, you can’t move your arms or legs, but you can move your eyes. Your breathing may also become faster. Scientists believe that these physical changes are related to the release of chemicals in your brain that help you remember and process what happens during the day. Sleep researchers also believe that these chemical changes in the brain play an important role in memory, learning and problem solving.