Transition from dim to bright light in the morning induces an immediate elevation of cortisol levels.
Level 3 - non-randomized controlled study
Within-subject crossover laboratory study without reported randomization in the abstract
PubMed 11231993 · doi:10.1210/jcem.86.1.7102
What was done
Eight healthy men completed three separate 36-hour protocols of continuous wakefulness under constant dim light, interrupted by either: continuous dim light throughout (baseline), a 3-hour pulse of bright light from 0500-0800 h (early morning), or a 3-hour pulse of bright light from 1300-1600 h (afternoon). Blood was sampled every 15 minutes for 24 hours to measure melatonin, cortisol, and TSH concentrations. Alertness was evaluated by measuring lapses on two computerized vigilance-sensitive performance tasks.
What was found
Early morning bright light exposure suppressed melatonin secretion, induced an immediate, greater than 50% elevation of cortisol levels, and limited the deterioration of alertness normally associated with overnight sleep deprivation. Morning light had no detected effect on TSH profiles. Afternoon bright light exposure had no effect on either hormonal (melatonin, cortisol, TSH) or behavioral parameters.
Why it matters
This study demonstrates that bright light directly stimulates the human corticotropic axis in a time-of-day-dependent manner, offering a physiological mechanism for the alerting effects of morning light exposure.
Limits
The sample size was very small (n = 8) and restricted exclusively to healthy men. The abstract does not state whether the order of conditions was randomized. Testing was conducted during 36 hours of continuous wakefulness, so results may not fully generalize to normal rested sleep-wake schedules.
Cited by
- supports Morning bright light exposure resets the circadian rhythm, stimulates the cortisol response, and inhibits melatonin production.