Morning and Evening Blue-Enriched Light Exposure Alters Metabolic Function in Normal Weight Adults.
Level 2 - randomized trial
Individual randomized trial with baseline within-subject comparison
PubMed 27191727 · doi:10.1371/journal.pone.0155601
What was done
Nineteen healthy adults completed a 4-day inpatient protocol under dim light conditions (<20 lux). Participants were randomized to 3 hours of blue-enriched light exposure on Day 3 starting either 0.5 hours after waking (morning group, n = 9) or 10.5 hours after waking (evening group, n = 10). Day 2 in dim light served as baseline. Subjective hunger and sleepiness scales were collected hourly. Blood was sampled at 30-minute intervals for 4 hours during the light exposure period to measure glucose, insulin, cortisol, leptin, ghrelin, and calculate HOMA-IR and area under the curve (AUC).
What was found
The abstract reports direction of effects without specific numerical values. In both morning and evening groups, insulin total area, HOMA-IR, and HOMA-IR AUC were increased and subjective sleepiness was reduced with blue-enriched light compared to dim light. The evening group, but not the morning group, showed a significantly higher peak glucose value during blue-enriched light exposure compared to dim light. No other significant differences were found between the morning and evening groups.
Why it matters
This study demonstrates that acute exposure to blue-enriched light can alter human glucose metabolism and insulin resistance markers in real time, with evening exposure producing additional elevations in peak glucose.
Limits
The sample size is very small (n = 19 across two parallel arms), limiting statistical power. The abstract reports no numerical values, effect sizes, or confidence intervals. The study only evaluated acute 3-hour light exposure in a controlled inpatient environment, leaving long-term metabolic impacts unmeasured.