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 the proliferation of neuroscience research from the second half of the twentieth century.
The importance of sleep is demonstrated by the fact that organisms daily spend hours of their time in sleep, and that sleep deprivation can have disastrous effects ultimately leading to death in animals. 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". However, the development of improved imaging techniques like EEG, PET and fMRI, along with faster computers have led to an increasingly greater understanding of the mechanisms underlying sleep.
The fundamental questions in the neuroscientific study of sleep are:
- What are the correlates of sleep i.e. what are the minimal set of events that could confirm that the organism is sleeping?
- How is sleep triggered and regulated by the brain and the nervous system?
- What happens in the brain during sleep?
- How can we understand sleep function based on physiological changes in the brain?
- What causes various sleep disorders and how can they be treated?
Introduction
, non-rapid eye movement sleep, and waking represent the three major modes of consciousness, neural activity, and physiological regulation. NREM sleep itself is divided into multiple stages: N1, N2 and N3. Sleep proceeds in 90-minute cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. As humans fall asleep, body activity slows down. Body temperature, heart rate, breathing rate, and energy use all decrease. Brain waves slow down. The excitatory neurotransmitter acetylcholine becomes less available in the brain. Humans often maneuver to create a thermally friendly environment—for example, by curling up into a ball if cold. Reflexes remain fairly active.REM sleep is considered closer to wakefulness and is characterized by rapid eye movement and muscle atonia. NREM is considered to be deep sleep, and is characterized by lack of prominent eye movement, or muscle paralysis. Especially during non-REM sleep, the brain uses significantly less energy during sleep than it does in waking. In areas with reduced activity, the brain restores its supply of adenosine triphosphate, the molecule used for short-term storage and transport of energy. During slow-wave sleep, humans secrete bursts of growth hormone. All sleep, even during the day, is associated with the secretion of prolactin.
According to the Hobson & McCarley activation-synthesis hypothesis, proposed in 1975–1977, the alternation between REM and non-REM can be explained in terms of cycling, reciprocally influential neurotransmitter systems. Sleep timing is controlled by the circadian clock, and in humans, to some extent by willed behavior. The term circadian comes from the Latin circa, meaning "around", and diem or dies, meaning "day". The circadian clock refers to a biological mechanism that governs multiple biological processes causing them to display an endogenous, entrainable oscillation of about 24 hours. These rhythms have been widely observed in plants, animals, fungi and cyanobacteria.
Correlates of sleep
One of the important questions in sleep research is clearly defining the sleep state. This problem arises because sleep was traditionally defined as a state of consciousness and not as a physiological state, thus there was no clear definition of what minimum set of events constitute sleep and distinguish it from other states of partial or no consciousness. The problem of making such a definition is complicated because it needs to include a variety of modes of sleep found across different species.At a symptomatic level, sleep is characterized by lack of reactivity to sensory inputs, low motor output, diminished conscious awareness and rapid reversibility to wakefulness. However, to translate these into a biological definition is difficult because no single pathway in the brain is responsible for the generation and regulation of sleep. One of the earliest proposals was to define sleep as the deactivation of the cerebral cortex and the thalamus because of near lack of response to sensory inputs during sleep. However, this was invalidated because both regions are active in some phases of sleep. In fact, it appears that the thalamus is only deactivated in the sense of transmitting sensory information to the cortex.
Some of the other observations about sleep included decrease of sympathetic activity and increase of parasympathetic activity in non-REM sleep, and increase of heart rate and blood pressure accompanied by decrease in homeostatic response and muscle tone during REM sleep. However, these symptoms are not limited to sleep situations and do not map to specific physiological definitions.
More recently, the problem of definition has been addressed by observing overall brain activity in the form of characteristic EEG patterns. Each stage of sleep and wakefulness has a characteristic pattern of EEG which can be used to identify the stage of sleep. Waking is usually characterized by beta and gamma depending on whether there was a peaceful or stressful activity. The onset of sleep involves slowing down of this frequency to the drowsiness of alpha and finally to theta of Stage 1 NREM sleep. This frequency further decreases progressively through the higher stages of NREM and REM sleep. On the other hand, the amplitude of sleep waves is lowest during wakefulness and shows a progressive increase through the various stages of sleep. Stage 2 is characterized by sleep spindles and K complexes. Stage 3 sleep has more sleep spindles. Stage 3 has very high amplitude delta waves and is known as slow wave sleep. REM sleep is characterized by low amplitude, mixed frequency waves. A sawtooth wave pattern is often present.
Ontogeny and phylogeny of sleep
The questions of how sleep evolved in the animal kingdom and how it developed in humans are especially important because they might provide a clue to the functions and mechanisms of sleep respectively.Sleep evolution
The evolution of different types of sleep patterns is influenced by a number of selective pressures, including body size, relative metabolic rate, predation, type and location of food sources, and immune function. Sleep is tricky behavior because it steeply increases predation risk. This means that, for sleep to have evolved, the functions of sleep should have provided a substantial advantage over the risk it entails. In fact, studying sleep in different organisms shows how they have balanced this risk by evolving partial sleep mechanisms or by having protective habitats. Thus, studying the evolution of sleep might give a clue not only to the developmental aspects and mechanisms, but also to an adaptive justification for sleep.One challenge studying sleep evolution is that adequate sleep information is known only for two phyla of animals- chordata and arthropoda. With the available data, comparative studies have been used to determine how sleep might have evolved. One question that scientists try to answer through these studies is whether sleep evolved only once or multiple times. To understand this, they look at sleep patterns in different classes of animals whose evolutionary histories are fairly well-known and study their similarities and differences.
Humans possess both slow wave and REM sleep, in both phases both eyes are closed and both hemispheres of the brain involved. Sleep has also been recorded in mammals other than humans. One study showed that echidnas possess only slow wave sleep. This seems to indicate that REM sleep appeared in evolution only after therians. But this has later been contested by studies that claim that sleep in echidna combines both modes into a single sleeping state. Other studies have shown a peculiar form of sleep in odontocetes. This is called the unihemispherical slow wave sleep. At any time during this sleep mode, the EEG of one brain hemisphere indicates sleep while that of the other is equivalent to wakefulness. In some cases, the corresponding eye is open. This might allow the animal to reduce predator risk and sleep while swimming in water, though the animal may also be capable of sleeping at rest.
The correlates of sleep found for mammals are valid for birds as well i.e. bird sleep is very similar to mammals and involves both SWS and REM sleep with similar features, including closure of both eyes, lowered muscle tone, etc. However, the proportion of REM sleep in birds is much lower. Also, some birds can sleep with one eye open if there is high predation risk in the environment. This gives rise to the possibility of sleep in flight; considering that sleep is very important and some bird species can fly for weeks continuously, this seems to be the obvious result. However, sleep in flight has not been recorded, and is so far unsupported by EEG data. Further research may explain whether birds sleep during flight or if there are other mechanisms which ensure their remaining healthy during long flights in the absence of sleep.
Unlike in birds, very few consistent features of sleep have been found among reptile species. The only common observation is that reptiles do not have REM sleep.
Sleep in some invertebrates has also been extensively studied, e.g., sleep in fruitflies and honeybees. Some of the mechanisms of sleep in these animals have been discovered while others remain quite obscure. The features defining sleep have been identified for the most part, and like mammals, this includes reduced reaction to sensory input, lack of motor response in the form of antennal immobility, etc.
The fact that both the forms of sleep are found in mammals and birds, but not in reptiles indicates that sleep might have evolved separately in both. Substantiating this might be followed by further research on whether the EEG correlates of sleep are involved in its functions or if they are merely a feature. This might further help in understanding the role of sleep in long term plasticity.
According to Tsoukalas, REM sleep is an evolutionary transformation of a well-known defensive mechanism, the tonic immobility reflex. This reflex, also known as animal hypnosis or death feigning, functions as the last line of defense against an attacking predator and consists of the total immobilization of the animal: the animal appears dead. The neurophysiology and phenomenology of this reaction show striking similarities to REM sleep, a fact which betrays a deep evolutionary kinship. For example, both reactions exhibit brainstem control, paralysis, sympathetic activation, and thermoregulatory changes. This theory integrates many earlier findings into a unified, and evolutionary well informed, framework.