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There is a variety of biological mechanisms in place that modulate levels of wakefulness and help to induce sleep at the appropriate times. Chief among these regulatory mechanisms is a person's circadian rhythm, commonly referred to as the 'biological clock'. Brain regions such as the reticular formation are able to maintain a general sense of the current time, and based on this information related brain areas will activate different internal systems at different times of the day. These systems in turn exert their effects by secreting hormones, changing cardiovascular function, altering metabolic rates, etc. Much of the circadian system's cyclical patterns are informed by the light levels it detects in the outside environment, such that exposure to artificial light may cause it to assume it is earlier than it actually is, and vice versa with artificial darkness.
In terms of sleep, the circadian rhythm most prominently exerts its effects by regulating the secretion of the hormone melatonin. In response to dimming light levels , the suprachiasmatic nucleus of the hypothalamus triggers the release of melatonin, which then begins a cascade of events leading to ever-increasing drowsiness and ultimately sleep. Although light is the most important direct cue, the continual breakdown of energy molecules like ATP and resulting accumulation of breakdown byproduct over the course of a day can also promote fatigue.
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To initiate sleep, brain activity has to be gradually slowed to the point at which it drops bellow the threshold of consciousness. This is accomplished by way of inhibitory neurons in the thalamus. Because the thalamus is also the central 'relay center' for sensory information to enter the higher (and more conscious) levels of the brain, this phenomena also helps explain the lack of conscious perception during sleep. The decline in brain activity is paralleled by other physiological changes, including slower heart rate, lower body temperature, and muscle relaxation. The end result of all these states is the entry into sleep.
Over the course of a night, brain activity fluctuates between several distinct phases. After declining down to the point of deep slow wave sleep, brain activity begins to cycle back up to a state which almost resembles a waking state. This phase is called REM sleep (short for Rapid Eye Movement, which is a peculiar characteristic of this phase). Physiological conditions like blood pressure surge back to approach waking levels as well, and activation of brain regions associated with motion forces the body to enter a state of temporary paralysis of the limbs. Eventually however, REM sleep will end the body will return to NREM sleep (short for Non-Rapid Eye Movement). Each progressive REM period becomes slightly longer as the body approaches its recommended amount of sleep, which, for reasons that will be discussed next, is essential both for feeling well-rested in the morning and for proper completion of the important biological functions going on during sleep.
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Although the potentially lethal effect of certain sleep disorders demonstrates the fact that sleep accomplishes a vital biological function, it remains somewhat unclear what exactly those essential functions are. The neuroscientific perspective of this question generally produces a handful of leading theories, chief among them is sleep's role in memory consolidation and learning.
As explained in the lesson on memory, for a memory to get consolidated into its long-term form, the synapses between neurons have to be chemically altered by repeated activation. Sleep, offers an excellent opportunity to complete this task, as new synaptic connections acquired during the day can be activated without interfering with any conscious functioning. This beneficial effect of sleep on memory has been demonstrated to extend not only to declarative memory, but to all other forms as well. In this concept also comes one of the most potent explanations for how dreams emerge (which, much like sleep's function itself, is still somewhat of a mystery). As some sort of neural network encoding a visual image, for example, is activated in the process of consolidating a related memory, this image may escape into a semi-conscious level of awareness, hence explaining the gripping yet surreal forms which tend to populate one's dreams.
However, dreams occur usually only during REM sleep, meaning that even if the theory is true, there still must be other purposes for sleep's other phases. Sleep can be a great opportunity for the body to complete a variety of other 'housekeeping' time functions, for the same reasons it is a good opportunity for memory consolidation. While the body is inactive, neurotransmitters depleted over the course of a day's use can be repleted, as can hormone levels. The ATP byproducts that triggered sleep in the first place can also be cleared away for the start of a new day.