Light intensity affects cognitive performance, alertness

Scientists at École Polytechnique Fédérale de Lausanne (EPFL)'s Solar Energy and Building Physics Laboratory (LESO; Lausanne, Switzerland) have proven that light intensity influences cognitive performance and alertness, and that these positive effects last until early evening. Their tests, using natural and artificial light, also confirm that light influences our subjective feeling of sleepiness.

Light synchronizes our biological clocks. It is collected in the eye by photoreceptors that use photopigments (pigments that change when exposed to light), known as melanopsin. 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.

The research team, led by Mirjam Münch, wanted to know how circadian rhythm could be influenced by perception of light during the daytime. To accomplish this, they created realistic office lighting conditions and recruited 29 young participants. "For this study, we took into account the intensity of natural and artificial light without specifically evaluating their spectra," explains Münch.

To synchronize their internal biological clocks, the volunteers had to maintain a regular sleep schedule during the seven days leading up to the test. They wore bracelets equipped with light sensors and accelerometers so that the scientists could monitor their movements.

The study itself took place over two eight-hour sessions. The participants spent the first six hours in an experiment room, first in well-lighted conditions (1000–2000 lux, more or less equivalent to natural light in a room). In the second session, the light intensity was about 170 lux, which is what the eye perceives in a room without a window, lit with artificial light. For this experiment, light intensity was measured at eye-level. Every 30 minutes, the subjects were asked to assess how alert or sleepy they felt.

Finally, at the end of each session, the participants underwent two hours of supplemental memory tests in a darkened room <6 lux. During the last two hours, the researchers took saliva samples in order to measure cortisol and melatonin concentrations. These two hormones are produced in a in a 24-hour cycle by the human body.

The volunteers who were subjected to higher light intensity during the afternoon were more alert all the way into the early evening. But when they were subjected to light intensity 10 times weaker, 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 their saliva. "With this study, we have discovered that light intensity has a direct effect on the subjective feeling of sleepiness as well as on objective cognitive performance, and that the benefits of more intense light during the daytime last long past the time of exposure," concludes Münch.

The work was recently published in Behavioral Neuroscience; for more information, please visit http://psycnet.apa.org/journals/bne/126/1/196/.

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