Six mornings of dim indoor light raised evening cortisol and disrupted sleep in healthy adults
Published Nov 1, 2025
Methods
Twenty healthy adults were randomly assigned to spend five mornings (8am to noon) under either low-intensity indoor lighting (55 lux, similar to typical dim indoor lighting) or brighter fluorescent lighting (800 lux) over a seven-day period. Researchers measured cortisol in saliva and urine across the day, and used sleep-lab recordings (polysomnography) on four nights to track sleep structure.
Findings
The dim-light group showed a sharp rise in evening cortisol and about 25 fewer minutes of total sleep compared with the brighter-light group, along with a shift in deep sleep timing toward later in the night — patterns that resemble those seen in people with depression.
Caveats & Context
This is a small study (20 people) over about a week, in healthy young adults with no diagnosed mood disorder — it shows a physiological pattern, not that dim indoor lighting itself causes depression, or that these short-term changes persist or scale up over months of typical indoor life. It also cannot say how this compares with real-world lighting, which varies far more than a controlled lab setting.
Living in biological darkness III: Effects of low-level pre-midday lighting on markers of depression in healthy subjects
Journal of Psychiatric Research · doi.org/10.1016/j.jpsychires.2025.11.008