We think of sleep as a restful time, but it’s hard to overstate its importance. During slumber, our muscles repair themselves, our brain sweeps out waste and forms memories. It boosts the immune system and regulates hormones that affect everything from appetite to stress levels. That’s why a good night’s rest is so crucial to our health. It’s no wonder, then, that those who regularly get less sleep have a higher risk for many common problems, including obesity, heart disease, depression and diabetes.

In his 1939 textbook, Sleep and Wakefulness, UChicago Prof. Nathaniel Kleitman laid out the foundation of what we know about sleep. His work with all-night electroencephalograms (EEG) allowed him to chart the changes that occur in the brain during the sleep cycle.

Researchers now know that sleep has a wide range of benefits for both the body and the mind. Getting enough high-quality sleep can help your immune system fight off diseases like the flu and colds, reduce your risk for developing heart disease and other chronic illnesses and improve your mood.

Mental Performance

Studies show that a lack of sleep can impair your ability to concentrate, pay attention and make decisions, and it may even lead to memory problems. When we’re sleep-deprived, the activity in parts of our brains that control emotions, such as the amygdala, decrease, making us irritable and prone to risk-taking behavior.

Tissue Growth and Repair

The release of proteins during slumber allows our bodies to build muscle, repair tissues and heal wounds. This is why it’s important for athletes to get enough sleep so they can recover from a strenuous workout.

Research suggests that sleeping can also help the body maintain a healthy weight by influencing the way we eat and our levels of hormones. People who get sufficient amounts of sleep tend to eat less and have lower levels of the hunger hormone, ghrelin.

Memory Consolidation

During non-rapid eye movement sleep and the deepest stage of sleep, called stage 3 and 4, your eyes and muscles move very little, and the brain waves slow down even more. When you reach the REM (rapid eye movement) phase of sleep, your breathing and heart rate increase, and you may begin dreaming. REM sleep is essential for remembering new information and experiences.

Researchers are still trying to learn exactly what happens at the cellular level during sleep, but they do know that this active state is necessary for brain plasticity. And they’re discovering that sleep plays an important role in our ability to process emotional and physical stimuli, and make good choices. Interestingly, a lack of sleep can change the way that certain genes function, which may explain why those who regularly sleep less tend to have more health problems such as heart disease and diabetes. That’s why getting adequate sleep is one of the American Heart Association’s “Life’s Simple 7,” along with moderate alcohol consumption, a balanced diet, physical exercise and not smoking.