A 30-minute morning light session cuts fasting glucose — and cortisol may explain why
STUDY: Fang J et al, Front Psychiatry 2026;17:1743465
STUDY TYPE: Randomized controlled trial
EVIDENCE GRADE: Moderate (7/10)
FUNDING: National Clinical Key Specialty Construction Project of China; Anhui Provincial Health and Wellness Scientific Research Project
Background
Light therapy treats depression by regulating the biological clock, and when that clock is off it can worsen diabetes and metabolic health. This study looked at whether light therapy also improves metabolic health — something antidepressants do not.
The Study
Forty-eight hospitalized adults with moderate-to-severe depression were randomized to full dose light therapy (10,000 lux) or a placebo (sham) light box with lower light (100 lux) light for 30 minutes each morning for two weeks. All continued antidepressants. Primary outcomes were fasting blood glucose and cortisol rhythm parameters (24-hour average cortisol levels, a stress hormone).
Light therapy significantly reduced fasting blood glucose compared to control. Depression scores (Hamilton Depression Scale) also improved more on light therapy. 24-hour average cortisol fell on light therapy, and that drop correlated with the drop in blood glucose. Lipid markers (ie, cholesterol) didn’t change.
Limitations: Small (24 per group), single-center, and only two weeks long. Men were overrepresented in the BLT group, and all participants were inpatients — a controlled environment that may not generalize to outpatient practice.
Practice Implications
- High cortisol and misaligned circadian rhythms are well known to contribute to depression. We now know that light therapy addresses both of those “root causes.”
- While this study adds to light therapy’s health benefits, it also carries risks for those with eye disease.
- Learn how to use light therapy here.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
What’s Your Take? Share in Comments
- Are you seeing benefits with light therapy? What gets in the way of using it?
