What is the Origin of Light Intensity Thresholds?

This concept originates from the psychophysics and chronobiology of light, particularly following the discovery of the intrinsically photosensitive retinal ganglion cells (ipRGCs). The "Threshold" refers to the point at which a stimulus is strong enough to produce a measurable physiological effect. Clinical application involves setting precise environmental light targets for different times of day.

What is the Mechanism of Light Intensity Thresholds?

The ipRGCs, containing the photopigment melanopsin, are the primary detectors for circadian regulation, responding most strongly to blue-spectrum light. A high-intensity, blue-rich light signal above the morning threshold directly activates the SCN, initiating the wake cycle and suppressing nocturnal melatonin. Conversely, light intensity below the evening threshold prevents unwanted SCN activation and allows for the natural, timely onset of melatonin secretion.

Light Intensity Thresholds

Meaning

The minimum specific illuminance levels, measured in lux or as melanopic equivalent daylight illuminance (mEDI), required to elicit a significant biological response in the non-visual photoreceptors of the human eye. Crossing these thresholds is necessary to either strongly suppress melatonin or effectively stimulate the central circadian pacemaker. Understanding and utilizing these thresholds is key to light-based chronotherapy and sleep optimization.