Sleep is a recurrent state of unconsciousness, in which the body’s activities are suspended for a period of time. It is important for human well-being, as it boosts muscle mass and repair of cells. The sleep-wake cycle is controlled by the biological clock. This complex neurochemical system is regulated by a number of hormones, including growth hormone, which promotes growth and repair of the body.
In addition to providing energy and boosting muscle mass, sleep is also believed to be beneficial for heart health. Scientists believe that this effect is related to the release of growth hormone, which is secreted by the pituitary gland. However, there are many questions surrounding the exact physiological processes involved in sleep.
For a human, the sleep cycle involves four stages. These are NREM (non-rapid eye movement) sleep, REM (rapid eye movement) sleep, and the transition from one stage to the next. REM sleep is associated with paralysis of most muscles, while breathing becomes faster. Stage 3 of the sleep cycle is referred to as slow-wave sleep, while stage 4 is called delta sleep.
Other mammals, such as mice and rats, exhibit recurrent periods of inactivity during sleep, though they are less well studied than humans. In some species, including lizards, it is not uncommon for an animal to be asleep for long periods of time. A study in rats showed that the repetition of nerve-signaling patterns during deep sleep is a form of memory encoding.
Despite the fact that humans and most other mammals show similar physiological correlates with sleep, some scientists still doubt whether sleep is a natural state. There are also several sleep disorders, such as insomnia and sleep apnea. Sleep deprivation can lead to a variety of health problems, such as high blood pressure, cardiovascular disease, and even epilepsy.
Many animals, such as fish, have a characteristic posture for sleep. They lie horizontally, which implies that the animal is passive towards the surroundings. Another characteristic of sleep is the absence of goal-directed behaviors. Some studies suggest that the first stage of sleep is characterized by slow, waveless activity, followed by a burst of rapid waves.
Rapid eye movement sleep, or REM, is a state of diffuse bodily activation. This is also known as paradoxical sleep. The EEG patterns for this phase of sleep are superficially similar to those of drowsiness. During this stage, eye muscles are still active, though they gradually relax. People in this state may also experience hypnic jerks, sudden muscle contractions. Although this is not necessarily cause for concern, it can be an indication of underlying sleep disorder.
REM sleep is believed to be the most effective way to prevent the spread of seizures, especially in people with epilepsy. Approximately three-quarters of human sleep time is spent in this type of sleep. It is estimated that about 40 percent of adults suffer from insomnia.
The four phases of sleep differ greatly in their duration and nature. Depending on the age of the individual, the body is ready for sleep at different times of the day.