Circadian Rhythms: Influence of Light in Humans

Many aspects of human physiology and behavior are dominated by 24 h rhythms that have a major impact on our health and well-being. For example, sleep– wake cycles, alertness and performance patterns, core body temperature rhythms, and the production of hormones such as melatonin and cortisol are all regulated by an endogenous near-24 h oscillator in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. The cells in these nuclei spontaneously generate rhythms with a period close to, but not exactly, 24 h, and in order for the circadian pacemaker to ensure that physiology and behavior are appropriately timed to anticipate events in the outside world, environmental time cues must be able to reset this internal clock. The major environmental time cue that resets these rhythms in mammals is the 24 h light–dark cycle generated by the earth’s axial rotation. Light information is captured exclusively by the eyes using specialized retinal photoreceptors and transduced directly to the SCN via a dedicated neural pathway, the retinohypothalamic tract (RHT). Each day the light–dark cycle resets the internal clock, which in turn synchronizes the physiology and behavior controlled by the clock. The major biochemical correlate of the light–dark cycle is provided by the pineal melatonin rhythm. Under normal light–dark conditions, melatonin is produced only during the night and provides an internal representation of the environmental photoperiod, specifically night length. The synthesis and timing of melatonin production requires an afferent signal from the SCN which projects to the pineal gland via the paraventricular nucleus and the superior cervical ganglion. Ablation of this pathway, as occurs in some people due to spinal damage at the upper cervical level, completely abolishes melatonin production, although other circadian rhythms not requiring this projection, such as the cortisol, temperature, and sleep–wake rhythms, persist. Light exposure during the night also inhibits melatonin production acutely and provides an indirect assessment of light input to the SCN via the RHT. Given the close temporal relationship between the SCN and melatonin production, the melatonin rhythm is often used as a marker of circadian phase and the melatonin suppression response as a proxy for RHT–SCN integrity and sensitivity.