Circadian Biology in Humans | Physiology of Circadian Rhythms
One of the most important adaptive features of organisms on earth is their ability to change their behavior and rest periods on a cyclic basis over a 24-hour period. The master circadian pacemaker in mammals was discovered in 1972 and, over the last 3 decades, rapid progress has been made in the discovery of the genetic and molecular components of the circadian clock.
In mammalian species, including humans, circadian rhythms are controlled by an oscillating pacemaker in the suprachiasmatic nucleus (SCN). The SCN is a miniscule collection of about 8,000 cells located in the anterior hypothalamus. The discovery that the SCN was the master circadian pacemaker resulted from the loss of various circadian rhythms following total destruction of the SCN in rats. These animals experienced total loss of rhythm in sleep-wake cycles, endocrine function, temperature regulation, and locomotor activity. Transplantation of fetal SCN tissue into SCN-lesioned animals, however, restored circadian rhythmicity. Surprisingly, the restored rhythm was that of the donor and not the host’s innate rhythm.
The SCN receives direct neural input from the retina via the retino-hypothalamic tract which relays light-dark information to the circadian clock. The SCN determines the individual’s free-running circadian period which, in man, is reported to average 24.2 to 24.5 hours in the absence of light cues. In normal humans, the phase of the SCN oscillator is entrained on a daily basis to the external light-dark cycle by information relayed through the retino-hypothalamic tract. In blind individuals who do not have a functioning retina or retino-hypothalamic tract, the circadian period can range from 23 to 25 hours.
In vertebrates, nocturnal versus diurnal activity has evolved as a means of species propagation. The distribution between periods of rest and activity varies between different animal species in order to minimize competition. For example, exothermic reptiles were active during the day, but endothermic mammals were nocturnal. Further evolution of the visual system in mammals, including most primates, as the retina developed color-sensitive cones (which are not required for night vision) led to a diurnal pattern of activity. Generally, nocturnal species have an innate circadian period of < 24 hours, whereas diurnal species have a period of slightly > 24 hours.