What is a Circadian Rhythm?

The system responsible for the generation and regulation of circadian rhythms is the circadian timing system. This neural system consists of a biological clock, input pathways, and output pathways. There is substantial evidence that the paired suprachiasmatic nuclei (SCN) in the anterior hypothalamus are the site of the biological clock. The SCN are located above the optic chiasm at the base of the third ventricle. The SCN exhibit endogenous rhythmicity and have a period of oscillation close to 24 hours. The hypothalamic suprachiasmatic neurons have 3 parts that maintain the inherent rhythmicities: a way to receive input from the environment to set the clock (light, temperature); the clock (a chemical timekeeping mechanism); and genes that help the clock control the activity of the other genes. The roles of circadian clock genes in human sleep disorders have been unclear. Variations in the expression of these genes result in alterations of the circadian time period and may account for individual differences in the timing of sleep periods and arousal.