Sleep is a naturally recurring state of mind and body, characterized by altered consciousness , relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement REM sleep,  and reduced interactions with surroundings. Sleep occurs in repeating periods , in which the body alternates between two distinct modes: REM sleep and non-REM sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual paralysis of the body. A well-known feature of sleep is the dream , an experience typically recounted in narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. During sleep, most of the body's systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the endocrine and immune systems. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.
In adults, and now in teens, poor sleep is linked to cardiovascular risk
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Sleep is a core behavior of adolescents, consuming up to a third or more of each day. As part of this special issue on the adolescent brain, we review changes to sleep behaviors and sleep physiology during adolescence with a particular focus on the sleeping brain. We posit that brain activity during sleep may provide a unique window onto adolescent cortical maturation and compliment waking measures. In addition, we review how sleep actively supports waking cognitive functioning in adolescence. Though this review is focused on sleep in healthy adolescents, the striking comorbidity of sleep disruption with nearly all psychiatric and developmental disorders for reviews see 1 , 2 further highlights the importance of understanding the determinants and consequences of adolescent sleep for the developing brain. Figure 1 illustrates the overarching themes of our review, linking brain development, sleep development, and behavioral outcomes. This diagram outlines the central theme of the review: a complex milieu of determinants and consequences of sleep interact during the sensitive window of adolescence.
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People with short sleep duration tend to weigh significantly more than those who get adequate sleep 1 , 2. The effect of sleep on weight gain is believed to be mediated by numerous factors , including hormones and motivation to exercise 4. Studies show that sleep-deprived individuals have a bigger appetite and tend to eat more calories. Sleep deprivation disrupts the daily fluctuations in appetite hormones and is believed to cause poor appetite regulation 2 , 5. This includes higher levels of ghrelin, the hormone that stimulates appetite, and reduced levels of leptin , the hormone that suppresses appetite 6.
Fragmented sleep — or simply not enough of it — has been linked to a higher risk of chronic health issues. Here a couple tosses and turns, as captured by Paul Maria Schneggenburger who uses time-lapse photography to examine sleep as part of his ongoing art project "The sleep of the beloved. Did you sleep well last night? Some 50 million to 70 million Americans have a chronic sleep disorder such as insomnia or sleep apnea, a condition in which breathing repeatedly stops and starts throughout the night.