Humans spend approximately one-third of their lives sleeping, more time than is spent on any other single activity. Everyone needs sleep, and everyone loves that feeling of lying down and closing his or her eyes at the end of the day. But despite the fact that sleep is such a central part of everyone’s life, to many people, the basic “what,” “how,” and “why” of sleep remain a mystery.
What is sleep?
Sleep is a natural and temporary period of mental rest during which both the mind and body undergo a highly active process of internal restoration. It is characterized by a suspension of consciousness and by changes to many of the body’s physiological functions, such as brain wave activity, body temperature, breathing pattern, heart rate and blood pressure.
How do people sleep?
There are several factors, both biological (internal) and environmental (external), that control people’s sleep. Biologically, sleep and wakefulness are controlled by brain cells called neurons. For approximately sixteen hours of the day, the brain activates wake-promoting neurons, which inhibit their sleep-promoting counterparts. However, the longer people are awake, the more their brains release certain chemicals that inhibit the wake-promoting neurons, therefore allowing the sleep-promoting neurons to be active. This gradual release of chemicals results in people’s homeostatic (internally-regulated) sleep drive. Although people’s sleep drives will cause increased levels of sleepiness throughout the day, the drive alone is not strong enough to cause them to fall asleep.
The brain’s biological clock, made up of approximately 50,000 brain cells, synchronizes people’s sleep-wake cycle with earth’s night-day cycle, therefore programming them to remain awake and fall asleep at certain times of the day. When the brain receives a light signal from the eyes, the biological clock’s circadian alerting system releases signals that combat the sleep drive, causing people to stay awake. When the alerting system recognizes darkness or that the body is ready for sleep, it switches off these signals, allowing the sleep drive to take over and start the transition from awake to asleep.
Once the circadian alerting system allows the sleep drive to take over, people typically enter a period of non-REM (non-rapid eye movement) sleep, which consists of the first three stages of the sleep cycle. (Although non-REM sleep is officially categorized by three sleep stages, models that specify four stages are also often used.) The first stage of non-REM sleep, NREM1, is a period of very light sleep in which the transition from being awake to being asleep occurs. Then, during NREM2, outside stimuli are increasingly repressed and conscious awareness begins to fade completely. This stage is the official onset of sleep and is important for sleep-based memory consolidation and information processing. NREM3 is the final stage of non-REM sleep, as well as the deepest stage of sleep in the whole sleep cycle. In this stage, people are completely unaware of any environmental sounds or stimuli, but the brain still continues information processing and memory consolidation.
After NREM3, the brain enters a period of REM (rapid eye movement) sleep, the final stage in the sleep cycle. During this period, the body is entirely immobile except for the eyes and respiratory system. REM sleep stimulates the brain regions used in learning and allows the brain to review and discard the day’s memories. This is also the stage in which the majority of dreams occur.
Throughout the night, the brain cycles between the different stages of non-REM and REM sleep. On average, a person will have three to five sleep cycles a night, each lasting between 90 and 120 minutes. The cycle goes from NREM1 to NREM2 to NREM3, where it stays constant for a period of time before going back to NREM2 to NREM1 and finally to REM. After a period of REM sleep, the cycle starts over with NREM1.
Why do we sleep?
Sleep is key to a person’s health, both mentally and physiologically. After a good night’s sleep, people often feel more awake and are able to function adequately during the following day, whereas a lack of sufficient sleep can have a negative impact on the person’s mood, judgment and ability to concentrate. Sleep, therefore, not only impacts people’s health, but also their safety and quality of life. Additionally, sleep plays an instrumental role in the strength and maintenance of a person’s memory, and is a key part of his or her learning process. Although much of the exact science behind why people sleep still remains a mystery, it is indisputably a crucial part of everyone’s life—no less important than food and water.
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