Sleep

Physiology

• Sleep is a state of reduced mental and physical activity, in which consciousness is altered, and sensory activity is inhibited to a certain extent.
• During sleep, there is a decrease in muscle activity, and interactions with the surrounding environment are significantly reduced.
• While sleep is different 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, in which the body alternates between two distinct modes: REM sleep and non-REM sleep.
• REM sleep: Rapid eye movement + 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.

Clinic

• 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.
• Disturbed circadian rhythm: The use of artificial light has substantially altered humanity's sleep patterns. This disrupts the release of the hormone melatonin needed to regulate the sleep-cycle.

Brain waves in sleep

The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep. Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep. Each waveform maintains a different frequency and amplitude.

• Alpha waves are seen in resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down.
• Beta waves take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert. Alpha and beta waves are the only waves seen when a person is in an awake state.
• Gamma waves are seen when a person is highly focused on a task or using all their concentration.
• Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2.
• Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.

Non-REM and REM sleep

Sleep is divided into two broad types:

1. Non-rapid eye movement (non-REM or NREM)
2. Rapid eye movement (REM) sleep.

Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states.

• Non-REM sleep occurs first and after a transitional period is called slow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.

• REM and non-REM sleep alternate within one sleep cycle, which lasts about 90 minutes in adult humans. As sleep cycles continue, they shift towards a higher proportion of REM sleep.
• American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.
• There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.

Awakening

"Waking up" redirects here. For other uses, see Waking up (disambiguation).

Further information: Wakefulness and Ascending reticular activating system "The Awakening", an illustration to writing by Leo Tolstoy Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex.

On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0-1% during their nightly sleep while males are awake for 0-2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep.

Today, many humans wake up with an alarm clock; however, people can also reliably wake themselves up at a specific time with no need for an alarm. Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle. Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia.

Determinants of alertness after waking up include quantity/quality of the sleep, physical activity the day prior, a carbohydrate-rich breakfast, and a low blood glucose response to it.