Characterizing the temporal Dynamics of Melatonin and Cortisol Changes in Response to Nocturnal Light Exposure.
Level 2 - randomized trial
Randomized controlled trial evaluating hormone dynamics across three light conditions
PubMed 31873098 · doi:10.1038/s41598-019-54806-7
What was done
Twenty-one young healthy participants were randomized into three parallel groups (n = 7 per condition) during the biological night for 6.5 hours of light exposure: intermittent bright light (~9,500 lux; IBL), continuous bright light (~9,500 lux; CBL), or continuous dim light (~1 lux; VDL). Melatonin suppression and cortisol levels were tracked using high-frequency blood sampling.
What was found
Melatonin suppression began within 5 minutes under bright light. In the IBL group, suppression half-life was ~13 minutes (mean suppression ~40%), and recovery between stimuli was slower (half-maximal recovery rate ~46 minutes, mean recovery ~50%). In the CBL group, suppression half-life was ~18 minutes with no recovery until light ended. Cortisol showed a significant linear increase between the start and end of each IBL stimulus, whereas CBL produced a trimodal response with an initial linear activating phase, an exponential inhibitory phase, and an exponential recovery phase.
Why it matters
These findings detail the distinct temporal dynamics of melatonin and cortisol regulation under light exposure at night, showing that continuous versus intermittent exposure patterns elicit fundamentally different endocrine responses.
Limits
The sample size was small (n = 7 per arm) and restricted to young, healthy participants. Light intensity (~9,500 lux) was very high and may not generalize to typical residential nocturnal light levels.