You’ve likely noticed how much better you feel when you wake up after a good night’s sleep. That feeling isn’t just about a fresh energy boost – it also means that the body and brain have been well repaired and restored to work at their best. Sleep is a critical part of life, and it affects almost every tissue and system in the body, including the heart, lungs, hormones, metabolism, immune function, mood, and disease resistance.

The study of sleep dates back centuries, and researchers have worked hard to understand its importance for mental health and physical wellbeing. Scientists now know that during slumber, the brain is busy with processing information, linking events and feelings into memories and links – a process known as memory consolidation. Sleep also helps to strengthen the immune system, which is why people who have a chronic illness are often advised to get plenty of rest and relaxation.

There is no single criterion for what constitutes true sleep; instead, scientists use various behavioral and physiological criteria. Most commonly, sleep is defined by the onset of a period of inactivity with a characteristic posture (horizontal repose) and the absence of overt goal-directed behavior. The presence of certain electroencephalogram patterns in the brain during this time, as well as the absence of arousal and the presence of certain eye movements, are also used to confirm the state of sleep. In rare cases, these criteria are not satisfied, such as when a person sleepwalks or enters a hypnotic trance, but even without them, most observers can usually agree that the subject is in true sleep.

Although the benefits of a good night’s sleep are clear, many people still underestimate how important it is for their wellbeing. In fact, if you are chronically underslept, it may be a risk factor for a number of illnesses, including obesity, heart disease, depression and anxiety, high blood pressure, and more.

The reason is simple: sleep plays a vital role in almost all of the body’s major systems, and it is essential for optimal functioning. During slumber, the brain processes new information and old memories, muscles repair themselves, the nervous system recharges, and hormones help control emotions and regulate metabolism and the immune system. A variety of hormones are released during deep sleep, including growth hormones that help children grow and increase muscle mass, as well as chemicals that regulate puberty, fertility and sexual maturation.

During a good night’s sleep, the brain will have processed all of this information and stored it in a memory bank, ready for use at the appropriate moment. For this reason, people who are suffering from a memory loss may have a difficult time remembering things, as their brain is not able to make the necessary connections. Sleep deprivation is also associated with poor memory, and this problem can become a vicious circle: people who are struggling to remember things may not get enough sleep, which leads to further problems, and so on.