Why Your Afternoon Light Matters More Than You Think

Relationships between light exposure and aspects of cognitive function in everyday life

The paper asks a simple question with a messy real-world twist: in everyday life (not a lab), do differences in the light you experience relate to how alert you feel and how well you perform on basic cognitive tasks?

What the authors did

• Sample: 58 UK adults (convenience sample) tracked for ~7 days of normal life; subset of 41 also completed an optional in-lab light-sensitivity session.

• Light measurement: a wrist-worn device logged melanopic equivalent daylight illuminance (EDI) every 30 seconds (a proxy emphasizing the melanopsin/ipRGC pathway often implicated in alertness and circadian biology).

• Cognition in the wild: participants used a smartphone app repeatedly through the day to report subjective sleepiness and complete short tasks for vigilance (PVT), working memory (3-back), and visual search.

Main findings

1) Recent bright light is linked to feeling less sleepy and responding faster

After adjusting for time-of-day and sleep-related factors, higher light exposure in the preceding window (30–120 minutes) was associated with:

• Lower subjective sleepiness (strongest up to ~2 hours).

• Faster reaction times on vigilance and working memory tasks (most robust around a 30-minute window for vigilance; up to ~1.5 hours for working memory).

The effect sizes are modest but measurable:

• A 1 log-lux increase in melanopic EDI corresponded to about a 0.2-point reduction in sleepiness score (on their scale).

• From very dim to bright outdoor conditions (a 4 log-lux span), vigilance reaction time improved by about 30 ms, and working-memory reaction time by about 60 ms.

2) Habitual “bright, stable days” correlate with better overall performance

Across the week, people with:

• Brighter daytime exposure (their “M10”: average brightness during the brightest 10 hours), and

• Less fragmented daily light patterns (lower intradaily variability),

tended to show better performance on several endpoints, including fewer errors (e.g., fewer false positives in visual search and working memory) and faster vigilance reaction times; key associations remained after adjusting for covariates.

3) Earlier “dark phase” and brighter days strengthened the light–sleepiness link

Participants with earlier timing of their dimmest-light period (used as an inferred proxy for earlier bed/rest timing) and higher daytime light showed a stronger relationship between recent light and reduced sleepiness.

4) Lab “photosensitivity” tests did not cleanly predict real-world sensitivity

The authors tried to predict who is most “light sensitive” (cognitively) using in-lab melanopsin-related measures (pupil tests and psychophysics). Overall, these did not robustly predict the real-world light–cognition slopes.

How the authors interpret it

They argue the results support two overlapping ideas:

1. Acute bright light exposure is associated with arousal-like benefits (alertness and faster reactions).

2. Habitual patterns of brighter, more stable daytime light may support cognition indirectly, potentially by supporting circadian robustness and sleep homeostasis.

Limitations to keep in mind

• The sample largely excluded people with major circadian disruption (e.g., night shift work), so effects could differ in those populations.

• Small sample for probing individual differences (e.g., genetics/age effects on photosensitivity not explicitly controlled).

• Correlational, not an intervention study, so it cannot establish causality (bright light might improve performance, but lifestyle factors correlated with bright light could also be driving results).

Bidirectional associations between sleep and physical activity investigated using large-scale objective monitoring data

https://www.nature.com/articles/s43856-025-01226-6

What the researchers wanted to know

We hear two common health messages: “get enough sleep” and “be physically active.” But it is not obvious whether people can realistically do both, day after day—and whether sleep and activity truly influence each other in everyday life (not just in short experiments). (Nature)

What they did

The authors analyzed objective, long-term data from 70,963 adults using two consumer devices: an under-mattress sleep sensor and a wrist-worn activity tracker. The dataset covered January 2020 to September 2023, across 244 geographic regions, totaling about 28 million person-days/nights. (Nature)

They asked two main questions:

1. How many people routinely hit commonly cited thresholds for both sleep and activity?

2. On a day-to-day basis, does sleep predict next-day step count, and does step count predict next-night sleep (and are those relationships non-linear)? (Nature)

What they found

1) Very few people consistently achieve both targets

Using common benchmarks (roughly 7–9 hours of sleep and >8,000 steps/day), only 12.9% of participants routinely met both. At the other end, 16.5% combined short sleep (<7h) with a sedentary step profile (<5,000 steps/day). (Nature)

2) Sleep was a stronger predictor of next-day activity than the reverse

In the day-to-day modeling, sleep characteristics were meaningfully associated with next-day steps, while steps had little association with the following night’s sleep.

Key patterns:

• Sleep duration vs. next-day steps: In the unadjusted analyses, the curve peaked around ~6 hours, but when the authors accounted for time awake (a critical confounder—if you sleep less, you simply have more waking hours to accumulate steps), the peak shifted closer to ~7 hours and the differences got smaller. (Nature)

• Sleep efficiency (how much time in bed is actually spent asleep): Higher sleep efficiency predicted more next-day steps (e.g., ~25th to 75th percentile difference corresponded to ~+282 steps/day, smaller after adjusting for awake time). (Nature)

• Sleep onset latency (how long it takes to fall asleep): Taking longer to fall asleep predicted fewer next-day steps (again, somewhat smaller after adjusting for awake time). (Nature)

• Steps → next-night sleep: Associations existed in places, but were small and often largely disappeared after adjusting for wake time—leading the authors to argue that the practical impact of “more steps today = better sleep tonight” was limited in these data. (Nature)

How to interpret this (without over-reading it)

• The headline result is not “sleep 6 hours to be more active.” The paper’s own adjustment shows that much of the apparent “6-hour advantage” is explained by having more hours awake to accumulate steps; once you control for that, the peak moves toward ~7 hours and the effect size shrinks. (Nature)

• A more defensible takeaway is: sleep quality (especially efficient sleep and falling asleep more easily) is linked with being more active the next day, while simply piling on steps may not strongly shift your sleep that night (at least as captured by these devices and metrics). (Nature)

Important limitations to keep in mind

This is a large, impressive dataset—but it’s not a perfectly representative slice of humanity.

• Participants were self-selected consumers who bought Withings devices; the sample skewed toward developed regions and likely higher socioeconomic status, and in this dataset participants were predominantly male. (Nature)

• The sleep sensor tends to overestimate sleep duration and time-to-fall-asleep compared with gold-standard lab sleep studies; if anything, the authors suggest this could mean the share truly meeting sleep recommendations may be even lower than reported. (Nature)

• “Activity” was measured as steps, which misses other meaningful movement (e.g., cycling, swimming, resistance training). (Nature)