Circadian rhythmicity and photobiological mechanisms of light sensitivity and discomfort glare in humans

Discomfort glare is a common visual sensation, which is generally reported when being exposed to a brighter lit environment. In certain clinical conditions, this sensation is abnormally amplified, and is commonly named photophobia. Despite the frequent appearance of this sensation in everyday life or in pathological conditions, the underlying mechanisms remain poorly understood. We show here, in highly controlled laboratory constant routine conditions, that light-induced discomfort glare is rhythmic over the 24-hour day. We reveal a strong circadian drive, with a sinusoidal rhythmicity, with maximal discomfort glare in the middle of the night and minimal in the afternoon. We also find a modest sleep-related homeostatic drive of visual discomfort, with a linear increase in discomfort glare over 34 hours of prolonged wakefulness. Our study reveals that discomfort glare is primarily driven by the ipRGC pathway, and that mid and/or long wavelengths cones are involved as well. The 6.5-hour phase lag between the rhythms of photoreceptors sensitivity, assessed through pupillary light reflex, and of glare discomfort, suggests two independent underlying mechanisms. In conclusion, our findings highlight the need to take time-of-day and biological rhythmicity into account in the evaluation of light-induced discomfort glare. Apprehending these mechanisms may help understand photophobia in clinical populations, such as in migraine patients, and should be taken into account to optimize light quality at home and at the workplace, both for day and night work.