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Sleep is differing from wakefulness by a lessor ability to react to stimuli
It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of being comatose.
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Sleep
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Sleep and memory
The relationship between sleep and memory has been postulated and studied since at least the early 19th century. Memory, the cognitive process whereby experiences, learning and recognition are recalled, is a product of brain plasticity, the structural changes within synapses that create associations between stimuli. Stimuli are encoded within milliseconds; however, the long-term maintenance of memories can take additional minutes, days, or even years to fully consolidate and become a stable memory (more resistant to change or interference). Therefore, the formation of a specific memory occurs rapidly, but the evolution of a memory is often an ongoing process.
Neuroscience of sleep
The neuroscience of sleep is the study of the neuroscientific and physiological basis of the nature of sleep and its functions. Traditionally, sleep has been studied as part of psychology and medicine. The study of sleep from a neuroscience perspective grew to prominence with advances in technology and proliferation of neuroscience research from the second half of the twentieth century. The fact that organisms daily spend hours of their time in sleep and that sleep deprivation can have disastrous effects ultimately leading to death, demonstrate the importance of sleep. For a phenomenon so important, the purposes and mechanisms of sleep are only partially understood, so much so that as recently as the late 1990s it was quipped: "The only known function of sleep is to cure sleepiness".
Sleep
Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of being comatose. Sleep occurs in repeating periods, in which the body alternates between two distinct modes known as non-REM and REM sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual paralysis of the body.
Sleep in non-human animals
Sleep in non-human animals refers to a behavioral and physiological state characterized by altered consciousness, reduced responsiveness to external stimuli, and homeostatic regulation. Sleep is observed in mammals, birds, reptiles, amphibians, and some fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep daily at night in diurnal organisms (such as humans) and in the day in nocturnal organisms (such as rodents). However, sleep patterns vary widely among species. Sleep appears to be a requirement for all mammals and most other animals.
Effects of sleep deprivation on cognitive performance
It has been estimated that over 20% of adults suffer from some form of sleep deprivation. Insomnia and sleep deprivation are common symptoms of depression and can be an indication of other mental disorders. The consequences of not getting enough sleep could have dire results; not only to the health of the individual, but those around them as sleep deprivation increases the risk of human-error related accidents, especially with vigilance-based tasks involving technology. The parietal lobes of the brain are largely involved in attention. Lesions to this region of the brain in humans result in difficulty or inability to attend to events that are contralateral to the lesioned hemisphere.
Rapid eye movement sleep
Rapid eye movement sleep (REM sleep, REMS) is a unique phase of sleep in mammals and birds, distinguishable by random/rapid movement of the eyes, accompanied with low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. The REM phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep because of physiological similarities to waking states, including rapid, low-voltage desynchronized brain waves. Electrical and chemical activity regulating this phase seems to originate in the brain stem and is characterized most notably by an abundance of the neurotransmitter acetylcholine, combined with a nearly complete absence of monoamine neurotransmitters histamine, serotonin, and norepinephrine.
Reticular formation
The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. The reticular formation is not anatomically well defined because it includes neurons located in diverse parts of the brain. The neurons of the reticular formation make up a complex set of networks in the core of the brainstem that stretches from the upper part of the midbrain to the lower part of the medulla oblongata. The reticular formation includes ascending pathways to the cortex in the ascending reticular activating system (ARAS) and descending pathways to the spinal cord via the reticulospinal tracts of the descending reticular formation.