photoentrainment

Photoentrainment is the process by which an organism's endogenous circadian rhythms are synchronized to the external light-dark cycle. In mammals, the retina detects environmental light and conveys signals to the brain's master clock, the suprachiasmatic nucleus (SCN). The SCN coordinates daily rhythms in physiology and behavior, aligning them with day and night.

Key photoreceptors are intrinsically photosensitive retinal ganglion cells (ipRGCs) that express melanopsin. These cells respond to light independently of vision and project to the SCN via the retinohypothalamic tract. Light exposure at different circadian times can advance or delay rhythms, as described by circadian phase-response curves. Short-wavelength blue light is particularly effective at driving entrainment due to melanopsin sensitivity, though rod-cone input also contributes.

In humans, photoentrainment influences sleep timing, hormone secretion (e.g., melatonin), body temperature, and alertness. Disruptions in entrainment—such as jet lag or shift work—can cause sleep disturbance, cognitive impairment, and mood symptoms. Therapeutic use of timed light exposure (phototherapy) or darkness strategies can support entrainment; blue-blocking filters and controlled lighting may also help in the evening.

Beyond humans, photoentrainment encompasses light-driven synchronization in other species and contexts. In plants, photoperiodism governs flowering. In experimental settings, light-dark schedules are used to study circadian physiology, including models in rodents and other nocturnal animals.

Measurement approaches include actigraphy, melatonin onset (dim-light melatonin onset), core body temperature, and hormonal assays. Photoentrainment is a central topic in chronobiology and sleep medicine, reflecting how environmental light shapes biological timing.