We know that bright light therapy works for seasonal depression, but with the non-seasonal one, results are still not conclusive, researchers from Cambridge said in 2016. And offered the conclusion by calculating the total effect across all available studies.


  • Generally, bright light therapy was effective. The effect size 0.41 represents a “small to moderate effect of light therapy in improving symptoms of depression in comparison with placebo/control treatment.” For a depression treatment without any side effects, this is certainly a success.
  • The effect was higher for morning bright light therapy (g=0.50). Today, we know that morning light works better, and is the standard for depression treatment.
  • The effect was much larger (g=0.63) when bright light was applied as a stand-alone therapy, without any other treatment and lower (g=0.25) for bright light together with other treatment. That’s a natural result of how treatments work, as we explain here.
  • Also, the effect was higher in out-patients (g=0.50) than those treated in hospitals (g=0.25). This is likely to reflect the difference between serious, hard-to-treat depressions which required pharmacotherapy and/or hospital treatment and lighter depressive states which respond to treatment better. Also, bigger differences between the inpatients make the effect size estimate look smaller, even if the actual effect remains the same.
  • Studies using placebo dim-light resulted in smaller effect of the real light (0.27) compared to those using other means of placebo (0.60). Most likely, this means that the lights triggered more efficient placebo healing effect than those “other means”. Which is good, because in the real world, placebo adds up to the biological effect.


There were large differences between the results of the individual studies. “The overall quality of evidence is poor due to high risk of bias and inconsistency,” the authors found. Partly, this may be due to differences in intensity of the light and the treatment: the study mixed everything calling itself “bright light therapy” together, not differentiating between the length of the treatment, light intensity and colour or anything else.

“Considering that light therapy has minimal side-effects and our meta-analysis demonstrated that a significant proportion of patients achieved a clinically significant response, light therapy may be effective for patients with non-seasonal depression and can be a helpful additional therapeutic intervention for depression.”

Perera, Rebecca Eisen, Meha Bhatt, Neera Bhatnagar, Russell de Souza, Lehana Thabane and Zainab Samaan (2016). Light therapy for non-seasonal depression: systematic review and meta-analysis. BJPsych Open, 2, 116–126.

Take-away message:

The effect of bright light therapy on depression in adults seems to be moderate. In studies published till 2015, there still was some confusion. First, some studies did not use the optimal high-intensity morning light, so they failed to confirm the full effect. Second, publication bias may have made the results look better than they really were. Taken both problems together, we can assume the healing effect of bright light therapy was confirmed, although more research was still needed in 2016. There is clinically meaningful effect and there seems to be no cost in terms of adverse effects which makes bright light therapy recommendable.


Studies included:

20 research studies; “Individuals with major depressive disorder (MDD) (single episode or recurrent), persistent depressive disorder (dysthymia), bipolar disorder with depression and clinically significant non-seasonal depressive symptoms were included. All formats of light therapy with regard to the timing of administration, the brightness and duration of light exposure, and the selected lighting device were included.”

Adverse effects:

Nothing significant. “The safety profile may be of particular interest for women with depression in the perinatal period where medications may be inappropriate or ineffective,” conclude the authors.