Sleep is a state of reduced mental and physical activity in which consciousness is altered and certain sensory activity is inhibited. During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment. While sleep differs 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.
Jet lag is a physiological condition that results from alterations to the body's circadian rhythms caused by rapid long-distance trans-meridian travel. For example, someone flying from New York to London, i.e. from west to east, feels as if the time were five hours earlier than local time, and someone travelling from London to New York, i.e. from east to west, feels as if the time were five hours later than local time. The phase shift when traveling from east to west is referred to as phase-delay of the circadian circle, whereas going west to east is phase-advance of the circadian circle. Most travelers find that it is harder to time zone adjust when traveling to the east. Jet lag was previously classified as one of the circadian rhythm sleep disorders.
A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism and responds to the environment. Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual. Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek χρόνος, and biology, which pertains to the study, or science, of life. The related terms chronomics and chronome have been used in some cases to describe either the molecular mechanisms involved in chronobiological phenomena or the more quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is required.
Biological rhythms are repetitive biological processes. Some types of biological rhythms have been described as biological clocks. They can range in frequency from microseconds to less than one repetitive event per decade. Biological rhythms are studied by chronobiology. In the biochemical context biological rhythms are called biochemical oscillations.
The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes. Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned.
The arcuate nucleus of the hypothalamus is an aggregation of neurons in the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence. The arcuate nucleus includes several important and diverse populations of neurons that help mediate different neuroendocrine and physiological functions, including neuroendocrine neurons, centrally projecting neurons, and astrocytes. The populations of neurons found in the arcuate nucleus are based on the hormones they secrete or interact with and are responsible for hypothalamic function, such as regulating hormones released from the pituitary gland or secreting their own hormones. Neurons in this region are also responsible for integrating information and providing inputs to other nuclei in the hypothalamus or inputs to areas outside this region of the brain. These neurons, generated from the ventral part of the periventricular epithelium during embryonic development, locate dorsally in the hypothalamus, becoming part of the ventromedial hypothalamic region. The function of the arcuate nucleus relies on its diversity of neurons, but its central role is involved in homeostasis. The arcuate nucleus provides many physiological roles involved in feeding, metabolism, fertility, and cardiovascular regulation.
Ghrelin is a hormone primarily produced by enteroendocrine cells of the gastrointestinal tract, especially the stomach, and is often called a "hunger hormone" because it increases the drive to eat. Blood levels of ghrelin are highest before meals when hungry, returning to lower levels after mealtimes. Ghrelin may help prepare for food intake by increasing gastric motility and stimulating the secretion of gastric acid.
Neuropeptide Y (NPY) is a 36 amino-acid neuropeptide that is involved in various physiological and homeostatic processes in both the central and peripheral nervous systems. It is secreted alongside other neurotransmitters such as GABA and glutamate.
Non-24-hour sleep–wake disorder is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "chronic steady pattern comprising [...] daily delays in sleep onset and wake times in an individual living in a society". Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the light–dark cycle in nature. Although this sleep disorder is more common in blind people, affecting up to 70% of the totally blind, it can also affect sighted people. Non-24 may also be comorbid with bipolar disorder, depression, and traumatic brain injury. The American Academy of Sleep Medicine (AASM) has provided CRSD guidelines since 2007 with the latest update released in 2015.
A zeitgeber is any external or environmental cue that entrains or synchronizes an organism's biological rhythms, usually naturally occurring and serving to entrain to the Earth's 24-hour light/dark and 12-month cycles.
Interpersonal and social rhythm therapy (IPSRT) is an intervention for people with bipolar disorder (BD). Its primary focus is stabilizing the circadian rhythm disruptions that are common among people with bipolar disorder (BD). IPSRT draws upon principles from interpersonal psychotherapy, an evidence-based treatment for depression and emphasizes the importance of daily routine (rhythm).
In chronobiology, an infradian rhythm is a rhythm with a period longer than the period of a circadian rhythm, i.e., with a frequency of less than one cycle in 24 hours. Some examples of infradian rhythms in mammals include menstruation, breeding, migration, hibernation, molting and fur or hair growth, and tidal or seasonal rhythms. In contrast, ultradian rhythms have periods shorter than the period of a circadian rhythm. Several infradian rhythms are known to be caused by hormone stimulation or exogenous factors. For example, seasonal depression, an example of an infradian rhythm occurring once a year, can be caused by the systematic lowering of light levels during the winter.
Circadian rhythm sleep disorders (CRSD), also known as circadian rhythm sleep-wake disorders (CRSWD), are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be caused either by dysfunction in one's biological clock system, or by misalignment between one's endogenous oscillator and externally imposed cues. As a result of this mismatch, those affected by circadian rhythm sleep disorders have a tendency to fall asleep at unconventional time points in the day. These occurrences often lead to recurring instances of disturbed rest, where individuals affected by the disorder are unable to go to sleep and awaken at "normal" times for work, school, and other social obligations. Delayed sleep phase disorder, advanced sleep phase disorder, non-24-hour sleep–wake disorder and irregular sleep–wake rhythm disorder represents the four main types of CRSD.
Neuropeptide Y receptors are a family of receptors belonging to class A G-protein coupled receptors and they are activated by the closely related peptide hormones neuropeptide Y, peptide YY and pancreatic polypeptide. These receptors are involved in the control of a diverse set of behavioral processes including appetite, circadian rhythm, and anxiety.
A chronotype is the behavioral manifestation of underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. Eveningness and morningness are the two extremes with most individuals having some flexibility in the timing of their sleep period. However, across development there are changes in the propensity of the sleep period with pre-pubescent children preferring an advanced sleep period, adolescents preferring a delayed sleep period and many elderly preferring an advanced sleep period.
A chronobiotic is an agent that can cause phase adjustment of the circadian rhythm. That is, it is a substance capable of therapeutically entraining or re-entraining long-term desynchronized or short-term dissociated circadian rhythms in mammals, or prophylactically preventing their disruption following an environmental insult such as is caused by rapid travel across several time zones. The most widely recognized chronobiotic is the hormone melatonin, secreted at night in both diurnal and nocturnal species.
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.
A constant routine protocol is a common method used in human circadian rhythm research to study internally generated, or endogenous, circadian rhythms without the effect of external, or exogenous, influences. In the method, subjects are kept in constant conditions for at least 24 hours. These include constant light and temperature, as well as constant semi-recumbent posture. In addition, subjects' food intake is evenly distributed throughout the protocol, and subjects are typically not allowed to sleep for the duration. While in these conditions, subjects are often assessed for a number of variables of interest. Two of the most common and best understood of these variables are core body temperature and melatonin.
Sleep is known to play an important role in the etiology and maintenance of bipolar disorder. Patients with bipolar disorder often have a less stable and more variable circadian activity. Circadian activity disruption can be apparent even if the person concerned is not currently ill.
