Sleep may be one of the most overlooked aspects of health. It’s difficult to make a habit of getting enough, and we often don’t give it the priority we do with things like diet, exercise and stress management. Yet, research over the past two decades has finally begun to show that good sleep is essential for a host of life-support functions. These include the proper functioning of our immune system, hormone regulation, emotional and psychiatric health, and learning and memory. Scientists spend much of their waking hours trying to understand what’s happening in our brains and bodies during this critical time.

Researchers are still learning what makes us sleep, and some of their findings may be surprising to many people. They’re not sure what causes humans to fall into a restful state, but they know that during sleep our body temperature drops, and our heart and breathing slow down. The brain’s electrical activity also shifts, from a highly active state to a more deactivated state, with bursts of activity at times.

There are several theories about why we need to sleep, but the most widely accepted is that sleep is needed for energy conservation. During the slumbering stage, our metabolic rate is about 35 percent lower than during wakefulness. This allows the body to conserve energy for important activities such as running and fighting, but also for activities that require less energy such as learning, thinking, and regulating emotions.

Another theory is that we need to sleep because it’s essential for brain function. During REM (rapid eye movement) sleep, brain activation occurs in areas of the brain that are normally associated with controlling emotion, and it’s more similar to what happens during the waking state. This suggests that a part of the reason we get so moody when we don’t get enough sleep is that our emotional centers aren’t operating properly.

Some of the other functions of sleep include storing memories, which is why it can be so difficult to remember things we learn during the day. It’s also when the brain processes emotions, and that’s why people who don’t get adequate rest may have more difficulty coping with stress, anger and sadness. It also helps the body repair itself, and during sleep the immune system releases proteins that help to fight infections.

For most adults, 7 to 9 hours of sleep a night is considered optimal. So, just as you prioritize a nutritious diet, physical activity and avoiding smoking, make it a priority to prioritize quality sleep too. It might be the key to preventing disease and enhancing your overall well-being.