Melatonin is a hormone that plays an integral role in diurnal rhythms. It is hypothesized that the action of orexin changes the activity of the neurotransmitters involved in the regulation of sleep/wake states. Orexin, which is produced in the hypothalamus, is a neuropeptide that plays an important role in maintaining wakefulness. These neurotransmitters contribute to maintaining wakefulness and significantly decrease during REM sleep. The sleep–wake cycle is also affected by neurotransmitters released by reticular activating system (RAS) neurons, such as norepinephrine, acetylcholine, and serotonin. ![]() ![]() Increased GABA and decreased histamine release induce NREM sleep by deactivating the cortex and thalamus. These neurotransmitters have opposing actions on the sleep–wake cycle. In the forebrain and hypothalamus, neurons release gamma-aminobutyric acid (GABA) and histamine. Sleep is regulated by a variety of chemicals in the body. 1 REM sleep correlates with activities of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system in healthy humans. Later in the cycle, when the individual enters REM sleep, electrical activity in the brain increases, contributing to increased blood flow to the brain, changes in respiratory and cardiac rates, and dreaming. NREM sleep is subdivided into three stages in which brain activity, eye movement, and skeletal muscle tone progressively decrease, placing the individual in a deeper state of sleep. Sleep typically begins with the NREM phase, which is followed by the REM phase. People experience two forms of sleep: rapid eye movement (REM) and nonrapid eye movement (NREM).
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