Everyday light exposure shapes how alert and mentally sharp we feel outside the lab

By Priyanjana Pramanik, MSc.Reviewed by Susha Cheriyedath, M.Sc.Dec 18 2025

Tracking people’s light exposure in everyday life reveals that brighter, more stable daily light is linked to feeling less sleepy and responding faster on key cognitive tasks, highlighting light as a modifiable factor for mental performance.

Study: Relationships between light exposure and aspects of cognitive function in everyday life. Image Credit: VesnaArt / Shutterstock

In a recent study published in the journal Communications Psychology, researchers examined the influence of exposure to light on subjective sleepiness and cognitive performance in everyday life.

They found that people with higher habitual and recent light exposure had faster reaction times and reduced sleepiness. Brighter and more stable daily light exposure patterns were also associated with improved performance on several cognitive measures, although observed effects were modest at the individual assessment level.

Biological and Circadian Effects of Light

Light is a powerful environmental signal that regulates circadian rhythms, sleep, alertness, and cognition. Beyond enabling vision, light affects the brain through intrinsically photosensitive retinal ganglion cells (ipRGCs), which influence melatonin secretion, circadian timing, and arousal.

Laboratory research has consistently shown that bright light can acutely improve alertness and cognitive performance, whereas exposure to inappropriate light at night disrupts sleep and impairs next-day cognition. Cognitive abilities themselves also vary across the day due to circadian rhythms, sleep pressure, and individual differences in light sensitivity.

Real-World Relevance of Lighting Conditions

However, most evidence comes from studies conducted under carefully controlled laboratory conditions. This leaves uncertainty about whether such effects apply to real-world environments where exposure to light varies widely in stability, timing, and intensity.

In modern societies, people spend a significant proportion of their time under artificial indoor lighting, often with insufficient daytime brightness and excessive evening light exposure. Understanding how everyday light exposure affects cognition is therefore relevant for workplace performance, safety, education, and long-term brain health.

Study Hypotheses and Participant Sample

Researchers hypothesized that short-term increases in light exposure would improve cognitive performance. They further proposed that long-term patterns of light exposure would influence overall cognition and that individual sensitivity to light could be identified using real-world or laboratory-based measures.

The study followed 58 adults in the United Kingdom over seven days. Participants were employed, community-dwelling adults without diagnosed sleep disorders or recent intercontinental travel, allowing the study to capture natural variation in cognition and daily light exposure.

Measurement of Light, Sleep, and Cognition

Personal light exposure was continuously recorded using a wearable wrist-mounted sensor that measured biologically relevant light levels every 30 seconds. Participants followed their normal routines and were instructed not to alter their lighting behavior or sleep.

Cognitive performance and self-reported sleepiness were assessed using a smartphone application multiple times per day at participants’ discretion. Tasks included a psychomotor vigilance task, a three-back working memory task, a visual search task, and subjective sleepiness ratings. Sleep timing and duration were recorded daily.

A subgroup of participants also completed an optional laboratory session to assess melanopsin-driven light sensitivity using pupillometry and perceptual brightness tasks. Statistical analyses used linear mixed-effects models to examine associations between light exposure, sleep variables, time of day, and cognitive performance while accounting for repeated measures and individual differences.

Effects of Recent and Habitual Light Exposure

Participants completed more than 1,400 assessments of subjective sleepiness and over 1,300 cognitive task sessions, capturing wide real-world variation in alertness and performance. Subjective sleepiness showed strong daily rhythms, being highest shortly after waking and increasing again later in the day. Sleep duration modestly reduced daytime sleepiness, while time awake and time of day had smaller effects on objective cognitive performance.

Recent light exposure was consistently linked to reduced subjective sleepiness and faster reaction times. Higher light exposure in the previous 30 to 120 minutes was associated with lower sleepiness scores, quicker responses on vigilance tasks, and improved reaction times during working memory tasks. Although effect sizes were modest, associations remained robust after accounting for sleep and circadian timing.

Habitual light exposure patterns were also relevant. Participants who experienced brighter daytime light and more stable light exposure performed better on specific tasks, particularly visual search and vigilance. In contrast, laboratory-based measures of light sensitivity had limited ability to predict real-world cognitive responsiveness to light, although some laboratory measures were modestly associated with average task performance. Substantial individual differences were observed.

Conclusions, Strengths, and Limitations

This study provides real-world evidence that everyday light exposure influences cognitive performance and alertness outside laboratory settings. Recent bright light was associated with reduced sleepiness and faster responses, while long-term exposure to stable daytime light correlated with better task-specific performance. These effects were comparable in magnitude to those of time of day alone, highlighting light as a modifiable environmental factor.

A major strength of the study is its ecological validity, combining wearable light monitoring with repeated smartphone-based cognitive testing. However, the correlational design precludes causal inference, and the relatively small sample of working adults without diagnosed sleep disorders limits generalizability to populations with circadian disruption or sleep pathology.

Overall, the findings suggest that optimizing everyday light exposure through brighter days, dimmer nights, and stable daily patterns may support cognitive performance and alertness, with implications for work environments, education, and public health.