Bright light helps daytime sleepiness; dim light helps anxiety
STUDY: Shen Y et al, J Affect Disord 2026
STUDY TYPE: Randomized crossover trial
FUNDING: National Natural Science Foundation of China and other non-industry groups in China
Background
Light therapy modulates dopamine. It may help movement in Parkinson’s disease, but the evidence there is mixed. This trial tested both light and dim-light therapy on two common problems for Parkinson’s patients: Sleep and anxiety.
The Study
- 33 patients with Parkinson’s disease, stable on their medications.
- Randomized crossover design: one month of bright light therapy (10,000 lux) and one month of dim light therapy (200 lux), separated by a one-month washout, with order randomized.
- Light was administered one hour each in the morning and evening.
- Outcomes included clinical scales, polysomnography, and resting-state EEG.
Results
Both intensities improved subjective sleep quality on the Sleep scale (PSQI). But the two diverged on other outcomes.
Bright light therapy improved daytime sleepiness scores (ESS) by an average of 3.4 points, roughly twice the reduction seen with dim light (1.2 points; Cohen’s d = 0.76). Dim light did not significantly reduce daytime sleepiness.
The opposite pattern held for anxiety. Dim light reduced anxiety scores (HAMA) by 1.7 points (Cohen’s d = 0.57), while bright light produced no significant change.
Other studies find improvements in sleep quality with light therapy in Parkinson’s disease, but the objective improvements in sleep here did not reach statistical significance, possibly due to sample size.
Side Effects
Light therapy was well tolerated. One patient reported brief skin itching during dim light therapy that resolved on its own.
Limitations
Small size, cross-over design, unblinded.
Practice Implications
- Consider light therapy in Parkinson’s disease. Learn how to choose an effective box.
- This trial adds support to its benefits in fatigue, and other studies support broader improvements: depression, sleep disturbance, and possibly motor function.
- The anxiolytic effects of dim light here are speculative, but I’ll clarify that light therapy does not typically cause anxiety. In most trials, light therapy improves anxiety.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
