The effects of red and blue lights on circadian variations in cortisol, alpha amylase, and melatonin.
Level 3 - non-randomized controlled study
Within-subjects experimental laboratory study without explicit mention of randomization in the abstract.
PubMed 20652045 · doi:10.1155/2010/829351
What was done
Twelve subjects were kept continuously awake for 27 hours in a within-subjects experimental design. Participants periodically received one-hour corneal exposures of 40 lux of narrowband long-wavelength (red) light or narrowband short-wavelength (blue) light. Acute responses in the endocrine and autonomic systems were assessed by measuring circadian variations in cortisol, alpha-amylase, and melatonin.
What was found
The abstract reports no exact numerical values, effect sizes, or statistical confidence intervals. It states that only blue light suppressed nocturnal melatonin. In contrast, both blue and red light altered cortisol levels, while effects on alpha-amylase were described as less clear.
Why it matters
These findings suggest that the nonvisual photic pathway mediating nocturnal melatonin suppression may operate through mechanisms distinct from those regulating autonomic and cortisol responses to light.
Limits
The sample size was very small (n = 12). The abstract provides no quantitative data, exact exposure schedules, or baseline participant characteristics (such as age, sex, or health status), and does not specify whether light exposures were randomized or counterbalanced.