Brain arousal is stimulated by the circadian system during the day and sleep is usually stimulated at night. The rhythms are maintained in the suprachiasmatic nucleus (SCN), located in the anterior hypothalamus in the brain, and synchronized with the day/night cycle. Gene-transcription feedback loops in individual SCN cells form the molecular basis of biological timekeeping. Circadian phase<unk> s are dependent on the schedule of light exposure, the intensity, and previous exposure to light. Variations in exposure can advance or delay these rhythms. For example, the rhythms can be delayed due to light exposure at night. Photoreceptors located in the retina of the eye send information about environmental light through the retinohypothalamic tract to the SCN. The SCN regulates the pineal gland, which secretes the hormone melatonin. Typically, the secretion of melatonin begins two hours before bedtime and ends two hours prior to waking up. A decline in neuronal firing in the SCN is caused by the binding of melatonin to the MT1 and MT2 melatonin receptors. It is believed that the reduction in firing in the SCN stimulates sleep. While day-active individuals produce melatonin at night, night<unk> <unk> ers' production of melatonin is suppressed at night due to light exposure. Circadian misalignment Circadian misalignment plays a major role in<unk> . Circadian misalignment occurs when there is no complete adaptation to a night<unk>  schedule. The hormones cortisol and melatonin are an important part of the circadian rhythm. In circadian misalignment, cortisol and melatonin lack entrainment to a night oriented schedule and stay on a daytime schedule. Melatonin continues to peak at night during a<unk> <unk> ers awake time and decreases during a<unk> <unk> ers sleep time. Cortisol levels are lower during a<unk> <unk> ers awake time and remain higher during<unk> <unk> ers sleep time.
Shift work sleep disorder
Circadian rhythm sleep disorder
Central sleep apnea
Sleep apnea
Sleep disorder
Sleep deprivation
Obstructive sleep apnea
Sleep-related breathing disorder