Getting a good night’s sleep helps you wake up refreshed, ready to take on the day. A bad night’s sleep, however, can leave you feeling tired, down and unable to concentrate. Sleep is the body’s most important restorative process, and research has linked it to a number of conditions, including memory consolidation, supporting normal immune function, and healing after injury or illness.

In the past, scientists used to think that a person’s ability to distinguish between sleep and wakefulness was largely a matter of their mental state at the time they fell asleep. Today, scientists use a combination of behavioral, motor, sensory and physiological criteria to define sleep.

The importance of sleep is reflected in the fact that sleep is one of only four lifestyle factors included in the American Heart Association’s “Life’s Simple 7” list of health habits for cardiovascular disease prevention. (The others are adequate exercise, a nutritious diet, not smoking and controlling blood pressure, cholesterol and weight.)

Sleep is also critical for maintaining healthy metabolism and a strong immune system. The brain, during the sleep cycle, clears cellular and protein debris that can otherwise build up in the body, contributing to inflammation and cell death. Studies show that people who do not get enough sleep are at increased risk for obesity, cardiovascular disease and depression. In children and adolescents, not getting enough sleep can affect their mood, behavior and learning and increase the likelihood of mental health problems.

Scientists are still working to understand the many functions of sleep, though a consensus is emerging that it is necessary for promoting healthy functioning of all systems in the body. Some researchers think that the primary function of sleep is to conserve energy, allowing animals to avoid using precious metabolic resources during periods of the day and night when it would be inefficient or dangerous to hunt for food or escape from predators.

Other scientists point to the role of sleep in regulating the circadian rhythm — a 24-hour internal clock that triggers certain bodily functions, including sleep-wake cycles. The circadian rhythm is controlled by an area of the brain called the hypothalamus.

During the non-rapid eye movement stages of sleep, the brain shifts from the more active alpha waves to slower theta waves and finally the deep slow wave stage of sleep. During this phase, the brain is able to organize and consolidate memories, making it harder for them to be forgotten. This is why people who do not get enough sleep may forget things more easily. Sleep also appears to help maintain cognitive performance by enhancing the ability to learn new information and improve retention of existing knowledge. In addition, a good night’s sleep can increase creativity and promote the growth of new neurons. This is known as neural plasticity. These processes are dependent on a good supply of oxygen to the brain, which is provided by sleep. When oxygen levels in the brain decrease, mental function declines.