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Sleep is a necessary biological process of unconsciousness believed to play a role in rest, refreshment of the body and memory saving, and although its biological imperative is unknown, its necessity is known.
It is believed to be closely tied with the circadian rhythm, an approximately 24-hour internal biological cycle synchronized with light and dark perceived by the eyes. Most mammals sleep in the nighttime in a biological adaptation to conserve energy and to not waste energy finding prey or food, as most other mammals are sleep and inert during the night. Some mammals, nocturnal animals, however, are active during the nighttime and sleep during the day.
A deprivation of sleep often leads to uncharacteristic apathy, violent temperament changes, hallucinations, lack of concentration and memory losses.
Sleep can be electrically quantified by an electroencephalogram (EEG). An EEG electrode is placed on the skin of the head, and the electrical fields of large concentrations of cortical structures and a few subcortical structures are of sufficient diameter to register on the electrode's recorder.
Sleep is characterized by three electrical waves on:
- Alpha waves - Associated with relaxed consciousness, occur approximately 10 minutes before sleep. Fairly high frequency of 8-13 hertz.
- Delta waves - Associated with sleep, high amplitude and synchronous.
- Theta waves - Associated with deeper sleep, higher amplitude and synchronity.
During sleep, these electrical waves often have a set rhythm: baseline, spike, decline, underspike, increase, baseline. This is because of electrical activity in the molecular layer (lamina one) of the cortical structures, the closest structures to an EEG electrode and therefore those with the highest influence on the EEG recorder. Intracellular electrical activity is obscured by the plasma membrane, and can not be measured. The reciprocating electrical activity in the surrounding extracellular fluid, however, is what is measured.
The axons and dendrites of lower cortical structures are localized in the molecular layer. When there is an action potential, the efferent neuron, often a pyrimidal cell, takes a significant amount of charge to capacitate. The positive charges of the depolarizating current travel from the axon hillock to the synaptic cleft in the molecular layer. Because positive charges accumulate in the intracellular space, reciprocating negative charges accumulate in the extracellular space, and this is measured as a downspike in the EEG recorder.
This causes a voltage sink in the synaptic cleft, and positive charges flow from the cell soma to fill the sink and restore electrical equilibrium, causing an overspike, and eventually, all falls back to electrical equilibrium of baseline electrical activity in the extracellular spaces.
There are two main types of sleep: REM (rapid eye movement) and non-REM.
Non-REM Sleep is more prevalent than REM sleep, and is associated with bodily refreshment.
The body and brain are idle, and cortical activity is low.
- See main article: REM Sleep
REM Sleep is associated with deeper sleep, and is less common than non-REM sleep.
External somatosensory sensation is completely numbed out, however, cortical activity is high, almost approaching wakefulness - the associated somatosensory processing systems are still dis-synchronously active. Neuromodulation from the diffuse neuromodulatory system is also responsible for severing all descending motor information. Movement is typically not possible in REM sleep, although sleepwalking is possible in REM sleep with younger individuals.
Dreams often occur during REM sleep. Because different parts of the brain are active, and not synchronous, bizzare mental states arise that translate into repetitive and unusual thoughts that are dreams. Somatosensory areas are active, but are not recieving incoming sensory information, therefore one normally may feel sensation in one's dreams, but not associated with the surrounding environment.
Sleep is divided into ninety-minute cycles of transiently alternating REM and non-REM sleep.
The first sleep cycle is often non-REM. Initially, at the beginning of sleep, there is Stage One sleep, where persons lose consciousness in 2-5 minutes and begin to fall to sleep, and decreasing cortical activity is detected. However, because Stage One sleep is the forerunner to later sleep and occurs only every 2-3 hours of relaxed consciousness, someone being awoken from Stage One sleep will often be unable to sleep until 2-3 hours later. In Stage Two sleep, there is increasingly deepening sleep, with increasing synchronicity and decreasing frequency. It also is marked by K-complexes (high amplitude and prolonged spikes) and sleep spindles (high frequency bursts), electrical anomalies in the cyclical nature of electrical activity because of subcortical structure irregular activity. Stage Three sleep is even deeper, and is predominated with deeper sleep with by delta waves. Stage Four sleep transitions from the first sleep cycle into the second sleep cycle, which is probably non-REM. In the case of sucessive second cycle of non-REM sleep, the Stage Four of the first cycle will transition into the Stage Two of the second cycle.
During early and middle sleep, predominately non-REM sleep, Stage Three and Four activity increases in frequency, denoting succeedingly increasing cortical activity. There is a low probability of REM in early sleep, but as sleep progresses, non-REM sleep becomes less prevalent. Therefore, the longer the sleep, the larger the proportion of REM to non-REM sleep is found at the end. By the end of the sixth sleep cycle (approximately 9 hours of sleep), there is 50% REM sleep to 50% non-REM sleep.
Acetylcholine is secreted from a cholinergic pontine structure, norepinephrine is secreted by the locus curealis, and serotonin is secreted from the dorsal raphae. In wakefulness, all three structures disinhibit thalamus to promote cortical activity.
In non-REM sleep, all three structures are depressed, therefore, inhibiting the disinhibition of the thalamus to eventually depress overall cortical activity. In REM sleep, where there is higher cortical activity, the cholinergic pontine structure is active, which selectively activates areas of the cortex to inhibit motor and somatosensory areas of the cortex but to dually promote higher associative functions, presumably areas of the brain integrated with dreams.