Sleep is a recurrent state of the brain that helps the body to rest, regenerate, and recover from illness and stress. It also contributes to mental health and is important for memory, learning, and social functioning. Lack of sleep increases the risk of chronic diseases, including cardiovascular disease, diabetes, and obesity. A good night’s sleep can help to combat depression, stress, and other health issues.

In humans, sleep is divided into two stages: REM and Non-REM. During REM sleep, the brain becomes active, producing an electrical signal that travels throughout the body. During Non-REM sleep, the brain becomes relatively deactivated. People who spend more time in REM sleep have lower muscle tone and higher levels of motor inhibition. However, REM sleep does not cause serious health problems.

Sleep is a complex process that affects nearly every part of the body. Some of the changes are internally generated, while others are regulated by the external environment. Most changes in sleep occur in the brain, where thousands of neurons send signals to different parts of the body. Brain-imaging studies have revealed that the brain is active in different ways during various stages of sleep.

Unlike wakefulness, sleep is a recurrent state, meaning that it is repeatable. This makes the process more complex. Scientists can define sleep by criteria such as periodicity, recurrence, and spontaneity. These criteria have been used to define sleep in birds, reptiles, and mammals. Subjective evidence is another source of definition, but it can be at odds with the scientific approach to defining sleep.

The main criterion for defining sleep is the absence of goal-directed behaviour. This can be defined by the presence of a decrease in the activity of the locus ceruleus, a small region in the brainstem, which is believed to be the source of motor inhibition.

Sleep is characterized by a horizontal posture, which implies a passive, non-responsive role toward the environment. During sleep, a person may experience bursts of rapid waves and a hypnic jerk, a sudden muscle contraction. While the exact mechanisms of these events are still unclear, scientists believe that they may be related to the onset of REM sleep.

Although the physical and chemical changes in a person’s body during sleep are well-known, many aspects of sleep are not understood. These include the changes in hormone levels, the coordinated brain activity, and the relaxation of skeletal muscles. Many animals have developed sleep for various purposes. For example, lizards avoid being eaten when it is dark, and some search for food or water during the day. Other mammals, such as rats, sleep during some of the day.

In addition to the physiological and hormonal changes that occur during sleep, the body’s immune system is affected. If you don’t get enough sleep, your immune system may not work properly, which can lead to infections, cancer, and other health issues.

Among the factors that affect the human body are the level of melatonin, the hormone that makes people feel sleepy. Melatonin is increased during the evening and decreased during the morning. Melatonin has a number of functions, but one of the most important is helping to repair and grow the body.