For many people, sleep is the most important part of the day. It’s the time when our bodies and minds process everything that happened during the day and prepare to take on tomorrow. If you’re not getting enough sleep, your energy is low, you feel sluggish and your mood can suffer. For this reason, it’s critical to make sleep a priority and get the rest you need.

For decades, scientists believed that sleep was a passive state in which the body shut down and took a break. But research has changed that view, showing that sleep is an active, complex biological process. It’s an essential component of our physical and mental health, but we still don’t fully understand what happens during sleep or why it’s necessary.

During sleep, our muscles are relaxed, there is a reduction in activity in the brain and the body’s responses to the environment are inhibited. It’s these features that help distinguish sleep from other states, like hibernation or a coma. It also helps explain why it’s hard to describe the experience of sleeping in words. For example, the characteristic horizontal posture of human sleep and that of most animals reflects this reduced activity, as do the closed eyes and the lack of overt goal-directed behavior.

Scientists have several theories about why we sleep and what happens during sleep. One theory is that sleeping helps us save energy by reducing our activity level and inhibiting the activities that require more energy, like moving. Another is that it’s a response to stress. During sleep, our bodies release a hormone called cortisol that can help fight off stress. Getting enough sleep can lower your levels of cortisol and give you more energy throughout the day.

Studies have also shown that sleep plays an important role in memory and emotion. During sleep, new memories are downloaded from short-term memory to long-term memory and the glymphatic system clears away waste products that build up during the day. This allows the brain to work well when you wake up. In addition, during sleep, brain activity increases in areas that regulate emotions, so getting enough rest can support emotional stability.

For children and teens, sleep is also important for growth and development. During deep sleep, the body releases hormones that promote healthy growth, increase muscle mass, regulate puberty and fertility, and repair cells and tissues. In addition, during this stage of sleep, the immune system gets a chance to remember germs and other things that may cause illness so it can quickly respond to them.

Getting the right amount of sleep is critical to good health, but it can be difficult because everyone has different needs. Insufficient sleep is associated with a variety of unfavorable health consequences including obesity, heart disease and diabetes. It can also make certain psychiatric conditions worse, such as depression and anxiety.