EPFL researchers demonstrate that daytime light intensity improves alertness and cognitive performance into the early evening
Scientists at EPFL’s Solar Energy and Building Physics Laboratory (LESO) have shown that the intensity of light we experience during the day affects how alert we feel and how well we perform on cognitive tasks, with benefits that persist into the early evening. Led by Mirjam Münch, the research team tested the effects of daytime light exposure on subjective sleepiness and objective memory performance. Their results were published in the journal Behavioral Neuroscience.
How light affects the body and brain
Light does more than support vision: it synchronizes the body’s internal clock, or circadian rhythm. Specialized retinal cells containing the photopigment melanopsin — a group of photoreceptors distinct from rods and cones — absorb photons across the visible spectrum and are especially responsive to blue light. These cells do not form visual images; instead, they convey information about environmental illumination to brain systems that regulate sleep, alertness, and hormonal cycles. Understanding how daytime light exposure influences these processes is central to improving well-being and performance in indoor settings.
Exploring office lighting and daytime exposure
Münch and her colleagues set out to study how realistic indoor lighting affects alertness and cognitive function during the day. They recreated typical office lighting conditions and recruited 29 young adult participants. The study focused on differences in light intensity rather than on spectral composition; participants experienced intensities that approximate common indoor and natural-light conditions.
Study design: from daytime to dusk
To align participants’ biological clocks, volunteers kept a consistent sleep schedule for seven days prior to testing. They wore wrist devices fitted with light sensors and accelerometers so the research team could track activity and light exposure leading up to the sessions.
The experimental protocol comprised two eight-hour sessions on separate occasions. During each session participants spent six hours in a controlled environment under one of two lighting conditions: a higher-intensity setting of roughly 1,000–2,000 lux (comparable to well-lit indoor spaces receiving natural light) and a lower-intensity setting of about 170 lux (similar to an interior room lit by artificial sources without direct daylight). Light levels were measured at eye level. Every 30 minutes participants rated their own level of sleepiness and alertness.
After the six-hour light exposure period, participants completed two hours of additional memory tests in a dimly lit room (below 6 lux). During this phase, researchers collected saliva samples to measure concentrations of cortisol and melatonin — hormones that follow a daily rhythm and can reflect changes in circadian and stress-related physiology.
Findings: brighter daytime light supports sustained alertness and memory
Participants exposed to higher light intensities during the afternoon reported feeling more alert and demonstrated better performance on memory tasks later into the early evening. In contrast, those who spent the session in lower-intensity light reported greater sleepiness and achieved lower memory test scores. Notably, these improvements in subjective alertness and objective cognitive function occurred without measurable differences in saliva cortisol or melatonin between lighting conditions.
“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,” says Mirjam Münch. The dissociation between behavioral effects and salivary hormone changes suggests that light can act via neural pathways that influence alertness and cognition independently of immediate hormonal shifts.
Practical implications for workplaces and indoor environments
These findings underline the importance of adequate daytime lighting in offices, classrooms, and other indoor environments where sustained attention and memory are important. Providing sufficiently bright, well-distributed light during daytime hours — and allowing access to daylight when possible — may enhance alertness, support cognitive performance, and help maintain productivity into the evening. While spectral composition and timing also matter for circadian health, this study highlights how overall light intensity alone can produce meaningful benefits for daytime functioning.
Notes about this research
Written by Sandy Evangelista
Contact: Sandy Evangelista – Ecole Polytechnique Fédérale de Lausanne
Source: Ecole Polytechnique Fédérale de Lausanne press release
Image source: Neuroscience image adapted from press release image with credit to EPFL
Original research: “Effects of prior light exposure on early evening performance, subjective sleepiness, and hormonal secretion” by Mirjam Münch, Friedrich Linhart, Apiparn Borisuit, Susanne M. Jaeggi, and Jean-Louis Scartezzini, published in Behavioral Neuroscience (2012), Vol. 126(1).