Sleep is a state of reduced mental and physical activity in which consciousness is altered and certain sensory activity is inhibited. During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment. While sleep differs from wakefulness in terms of the ability to react to stimuli, it still involves active brain patterns, making it more reactive than a coma or disorders of consciousness.
Sleep occurs in repeating periods, during which the body alternates between two distinct modes: rapid eye movement sleep (REM) 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. Dreams are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep.
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 internal circadian clock promotes sleep daily at night, when it is dark. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep is a highly conserved behavior across animal evolution, likely going back hundreds of millions of years, and originating as a means for the brain to cleanse itself of waste products. In a major breakthrough, researchers have found that cleansing, including the removal of amyloid, may be a core purpose of sleep.
Humans may suffer from various sleep disorders, including dyssomnias, such as insomnia, hypersomnia, narcolepsy, and sleep apnea; parasomnias, such as sleepwalking and rapid eye movement sleep behavior disorder; bruxism; and circadian rhythm sleep disorders. The use of artificial light has substantially altered humanity's sleep patterns. Common sources of artificial light include outdoor lighting and the screens of electronic devices such as smartphones and televisions, which emit large amounts of blue light, a form of light typically associated with daytime. This disrupts the release of the hormone melatonin needed to regulate the sleep cycle.
A sleeve (Old English: slīef, a word allied to slip, cf. Dutch sloof) is the part of a garment that covers the arm, or through which the arm passes or slips.
The sleeve is a characteristic of fashion seen in almost every country and time period, across a myriad of styles of dress. Styles vary from close-fitting to the arm, to relatively unfitted and wide sleeves, some with extremely wide cuffs. Long, hanging sleeves have been used variously as a type of pocket, from which the phrase "to have up one's sleeve" (to have something concealed ready to produce) comes. There are many other proverbial and metaphorical expressions associated with the sleeve, such as "to wear one's heart upon one's sleeve", and "to laugh in one's sleeve".
Early Western medieval sleeves were cut straight, and underarm triangle-shaped gussets were used to provide ease of movement. In the 14th century, the rounded sleeve cap was invented, allowing a more fitted sleeve to be inserted, with ease around the sleeve head and a wider cut at the back allowing for wider movement. Throughout the 19th century and particularly during the Victorian era in Western culture, the sleeves on women's dress at times became extremely wide, rounded or otherwise gathered and 'puffy', necessitating the need for sleeve supports worn inside a garment to support the shape of the sleeve. Various early styles of Western sleeve are still found in types of academic dress.
Sleeve length varies in modern times from barely over the shoulder (cap sleeve) to floor-length (as seen in the Japanese furisode). Most contemporary shirt sleeves end somewhere between the mid-upper arm and the wrist.
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