The cells in these nuclei spontaneously generate rhythms with a period close to, but not exactly, 24 hours, 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 to 24 hours. The major environmental time cue that resets these rhythms in mammals is the 24-hour Inhibitors,research,lifescience,medical 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 Inhibitors,research,lifescience,medical 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 lightdark cycle is provided by the pineal melatonin rhythm.

Under normal light-dark conditions, melatonin is produced only ERK inhibitor during the night, and provides Inhibitors,research,lifescience,medical an internal representation of the environmental photoperiod, specifically nightlength. 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. Light exposure during the night also inhibits melatonin production acutely and provides an indirect assessment of light input to the SCN via the RHT (Figure 1).1-4 Given the close temporal relationship between the SCN and melatonin production, the melatonin rhythm is often Inhibitors,research,lifescience,medical used as a marker of circadian phase and the melatonin suppression response as a proxy for Inhibitors,research,lifescience,medical RHT-SCN-pineal integrity and sensitivity Figure 1. Neuroanatomy of the circadian system. Light is detected by specialized retinal photoreceptors and transduced

to the circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) via the monosynaptic retinohypothalamic tract (RHT). SCN efferents … The role of light in circadian entrainment Under normal first conditions, the vital importance of daily light-dark exposure to entrain circadian rhythms is taken for granted. For example, although not consciously perceived, the ability to sleep at night and be awake in the day is largely controlled by the circadian system, which promotes sleep during the night and wakefulness during the day via alterations in efferent signals from the SCN to other hypothalamic areas controlling sleep-wake states.5 The difficulties associated with trying to sleep during the day or be awake at night at the “wrong” circadian time is readily observed in sighted people following rapid transmeridian travel (“jet lag”)6 or in night-shift workers.