Light intensity has a direct impact on cognitive performance and alertness, and on the subjective feeling of sleepiness—and the benefits of more intense light last long past the time of exposure. In seeking to learn how circadian rhythm could be influenced by perception of light, research done at École Polytechnique Fédérale de Lausanne (EPFL)'s Solar Energy and Building Physics Laboratory (LESO; Lausanne, Switzerland) used natural and artificial light, taking into account the intensity of natural and artificial light without specifically evaluating their spectra.1
The researchers found that volunteers subjected to higher light intensity during the afternoon (1000–2000 lux, more or less equivalent to natural light in a room) were more alert all the way into the early evening than those who were not. But when subjected to light intensity 10 times weaker (about 170 lux, which is what the eye perceives in a room without a window, lit with artificial light), they showed signs of sleepiness and obtained lower scores on the memory tests. The results were observed even in the absence of changes in cortisol and melatonin concentrations in the volunteers' saliva.
Light synchronizes our biological clocks. It is collected in the eye by photoreceptors that use melanopsins—pigments that change when exposed to light. These cells, which differ from rods and cones, are considered a third class of photoreceptors in the retina and were discovered only 10 years ago. The cells also perceive and absorb photons in the visible light spectrum, and are stimulated by blue light.
1. M. Münch et al., Behavioral Neurosci., 126, 1, 196–203 (2012).