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What is Sleep?

Human life is divided into sleep and wake states. The need for sleep is unquestioned, yet the exact reasons for why we need sleep are not fully clear. Sleep is a state where the brain controls various physiological functions of the body and puts them at a relative rest in order to prepare them for proper and maximal function during wakefulness. During sleep, important substances in the brain and nervous system are replenished and byproducts, which are the result of brain activity, are cleared out. As such, sleep plays a major role in maintaining proper functioning of the brain. Sleep is an absolute necessity for crucial brain functions such as alertness, memory, reasoning, learning, reflexes, motor skills and a general sense of well-being. Additionally, adequacy or quality of sleep has a significant impact on other aspects of human health, including risk of heart attack and stroke, hypertension, diabetes prevention and control, and weight control.

  • Sleep Cycles

    During sleep, the brain flows through different sleep stages. There are two types of sleep: REM (rapid eye movement) and NREM (non-rapid eye movement). NREM sleep is composed of three numbered stages.

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    Stage 1 sleep lies between wakefulness and light sleep. It is the moment where people drift in and out of sleep and can be easily awakened. In this stage, the brain transitions from alpha waves to theta. Twitches and sudden jerks typically occur during this stage of sleep.

    Stage 2 sleep is a light sleep. It is characterized by a marked decrease in muscle activity and little to no awareness of the person's surroundings. This stage accounts for 45 to 55% of total sleep in adults.

    Stage 3 is known as delta, or slow-wave, sleep and is a stage of deep NREM sleep. This sleep stage is central for regulation of important hormones such as cortisol and growth hormones. During Stage 3, the body regenerates tissue, builds bone, and grows muscle during this stage.

    REM sleep is the stage of sleep in which dreaming occurs. It is characterized by rapid eye movements and increased brain activity. It accounts for 20-25% of total sleep time in adults. REM sleep is also associated with extreme instability of the respiratory and cardiac functions.

  • Sleep Timing - The Circadian Rhythm

    The circadian rhythm is an innate biological mechanism by which we are tuned to a 24-hour sleep-wake cycle. The rhythms are adjusted by the body's awareness of daylight and night. All life forms have cycles of rest and activity. In humans and higher animals, physiological functions are organized into the circadian cycle (from circa meaning about and dian meaning day). The circadian cycle is maintained by internal and external factors. Internal factors are genetically determined and are controlled by the supraoptic nucleus. This is a collection of cells deep in the brain which receives signals generated from external factors, specifically, light and dark (i.e. day and night). In turn, this controls the release of melatonin by the pituitary gland. The cycling of melatonin levels in correspondence with day and night promotes sleep at night and wakefulness during the day. The cycle also governs multiple physiological functions such as the secretion of hormones, gastrointestinal and cardiovascular functions, alertness and lethargy.

  • Circadian Cycle - How It Works

    Generally, the cycle dictates an increased tendency towards sleepiness in the late evening hours, which continues to increase throughout the night until early morning hours. Then, that sleepiness starts to decrease while there is a corresponding increase in the tendency to be awake and alert during the daytime hours. The rise of the wakefulness and decline of sleepiness continue until mid-afternoon, when for a brief period (shortly after lunch), sleepiness increases while alertness diminishes, until it reverses again until late afternoon. Therefore, there isn't a state of 100% sleep or 100% wakefulness. Instead, there is a majority of one or the other that work against each other like yin and yang. Other factors that regulate the sleep-wake cycle include clues and activities that relate to the time of day - routines such as breakfast, lunch and dinner, and getting ready for bed. These are collectively called zeitgebers - a German word that means "time setters".

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  • Disruptions in the Circadian Cycles

    The routine necessities of daily life - waking up at a set time in the morning, going to work at a scheduled time, and exposure to sunlight throughout the day - are potent factors in maintaining a normal circadian cycle. However, if these routines are disrupted by something such as a change in schedule, it can cause a disorder called phase delay syndrome. An example would be a summer recess for a high school student who progressively goes to bed later each night and sleeps later each morning, decreasing exposure to morning sunlight. As a result, when school resumes in the fall, the student will have trouble waking up timely each morning until their routine resets.