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Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from wakefulness by a decreased ability to react to stimuli, and is more easily reversible than being in hibernation or a coma.
Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in mammals, birds, reptiles, amphibians and fish, and in some form also in insects and even simpler animals such as nematodes (see the related article Sleep (non-human)).
The purposes and mechanisms of sleep are only partially clear and the subject of substantial ongoing research. Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%. Additionally it is observed that mammals require sleep even during the hypometabolic state of hibernation, in which circumstance it is actually a net loss of energy as the animal returns from hypothermiatoeuthermia in order to sleep.
Further information: Neural oscillationNREM stage 1: This is a stage between sleep and wakefulness. The muscles are active, and the eyes roll slowly, opening and closing moderately.
NREM stage 2: theta activityIn this stage, it gradually becomes harder to awaken the sleeper; in this stage the alpha waves of the previous stage are interrupted by abrupt activity called sleep spindles and K-complexes.[
NREM stage 3: Formerly divided into stages 3 and 4, this stage is called slow-wave sleep (SWS). SWS is initiated in the preoptiiarea and consists of delta activity, high amplitude waves at less than 3.5 Hz. The sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions.
REM: The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete serotonin. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage. Vital signs indicate arousal and oxygen consumption by the brain is higher than when the sleeper is awake.[An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. The function of REM sleep is uncertain but a lack of it will impair the ability to learn complex tasks. One approach to understanding the role of sleep is to study the deprivation of it.[.During this period, the EEG pattern returns to high frequency waves which look similar to the waves produced while the person is awake
Dreams are successions of images, ideas, emotions, and sensations that occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation and a subject of philosophical and religious interest throughout recorded history. The scientific study of dreams is calledoneirology. Scientists believe that all mammals dream, but whether this is true of other animals, such as birds or reptiles, is uncertain.Dreams mainly occur in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep. At times, dreams may occur during other stages of sleep. However, these dreams tend to be much less vivid or memorable.Dreams can last for a few seconds, or as long as 20 minutes. People are more likely to remember the dream if they are awakened during the REM phase. The average person has three to five dreams per night, but some may have up to seven dreams in one night. The dreams tend to last longer as the night progresses. During a full eight-hour night sleep, most dreams occur in the typical two hours of REM.
In modern times, dreams have been seen as a connection to the unconscious. They range from normal and ordinary to overly surreal and bizarre. Dreams can have varying natures, such asfrightening, exciting, magical, melancholic, adventurous, or sexual. The events in dreams are generally outside the control of the dreamer, with the exception of lucid dreaming, where the dreamer is self-aware. Dreams can at times make a creative thought occur to the person or give a sense of inspiration.Opinions about the meaning of dreams have varied and shifted through time and culture. Dream interpretationsdate back to 5000–4000 BC. The earliest recorded dreams were acquired from materials dating back approximately 5,000 years, in Mesopotamia, where they were documented on clay tablets. In the Greek and Roman periods, the people believed that dreams were direct messages from the gods or from the dead, and that they predicted the future. Some cultures practiced dream incubation with the intention of cultivating dreams that are prophetic.Sigmund Freud, who developed the discipline of psychoanalysis, wrote extensively about dream theories and interpretations. He explained dreams as manifestations of our deepest desires and anxieties, often relating to repressed childhood memories or obsessions. In The Interpretation of Dreams, Freud developed a psychological technique to interpret dreams and devised a series of guidelines to understand the symbols and motifs that appear in our dreams.