Meta-analytical studies gather all available research satisfying given criteria and calculate the overall results across the studies. “Bright light therapy appears to be efficacious,” concluded the authors of one such study from 2016.

Such conclusion is adequate given the data they obtained. Nevertheless, because they also pointed to three studies that did not reveal any effect of bright light therapy on depression, I wanted to understand what happened in these three cases.

Is bright light therapy inefficient in some groups of depressed people?” I was asking myself. But reading through the three pessimistic studies, I was relieved: the studies were clearly problematic.

  • The first one is concerned with post-partum depression. And indeed, it found no effect of bright light therapy – because they only had 5 mothers in the control group. To understand the problem, imagine you were interested in whether men are taller than women and compared five gentlemen to ten ladies. You would fail to prove anything because the sample size is too small to say conclusively whether it is only the selected sample that is taller or whether there is evidence of a trend. The same happened in this study: the sample was simply too small.
  • The second study did not look at major depression but bipolar disorder instead. Maybe it plays a role, maybe not. Anyway, it does not prove anything about major depression and thus is irrelevant to this research question.
  • Finally, the third study was weak because it lasted only seven days. Despite that after one week patients receiving bright light therapy were improving, the period was too short to rule out a random influence and thus cannot be used to prove statistical significance.

Plus, out of hundreds of studies on the topic, the meta-analysis includes only 9 studies that satisfied defined criteria. Of course, they needed to comply to their measures and only accept placebo-controlled studies. However, this eliminates all other studies showing that bright light therapy is as good or even better than some other treatment, including our favourite study.

Taken together:

“Bright light therapy seems efficacious in improving nonseasonal depression,” conclude the authors.

Al-Karawi, Jubair (2016). Bright light therapy for nonseasonal depression: Meta-analysis of clinical trials. Journal of Affective Disorders, Volume 198, 1 July 2016, Pages 64-71

….and the actual evidence available in 2019 is even stronger than that.