> the small sample size of the only available study
The only study?
https://scholar.google.com/scholar?q=light+therapy+depressio...
There are numerous studies showing a benefit from light therapy.
It is true that the the quality of the evidence could be improved. That's a political/financial issue, not a reflection of the efficacy (or lack thereof) of light therapy. The available evidence supports the treatment.
Basically when you say "there are studies that show X" you aren't saying anything because "studies" are cheap unless your methodology is rigorous and sound, which isn't the case a lot of the time.
http://archpsyc.jamanetwork.com/article.aspx?articleid=20429...
http://ajp.psychiatryonline.org/doi/full/10.1176/ajp.2006.16...
http://archpsyc.jamanetwork.com/article.aspx?articleid=20433...
Apparently, 91 papers passed Nussbaumer et al's assessment of the abstract (and were presumably randomized, controlled trials):
We assessed 91 full-text papers for inclusion in the review, but only one study providing data from 46 people met our eligibility criteria.
Those eligibility criteria were:
For efficacy, we included randomised controlled trials on adults with a history of winter-type SAD who were free of symptoms at the beginning of the study.
I think it may have been the focus on prevention (rather than treatment of symptomatic depression) that excluded all the other papers.
I just skimmed the paper the paper the first time and didn't notice that the authors were searching for studies of a very specific question: prevention (excluding treatment) of seasonal affective disorder (excluding any other form of depression).
http://www.cochrane.org/CD011269/DEPRESSN_light-therapy-prev...
> Which studies were included in the review?
> We searched databases up to August 2015 for studies on light therapy to prevent winter depression. Among 2986 records, we found one randomised controlled study including 46 people who received light therapy or no treatment. All individuals in these studies had a history of winter depression.
> What does the evidence from the review reveal?
> The quality of evidence for all outcomes was very low, so we can draw no conclusions about whether light therapy is effective in preventing winter depression. The included study provided no information on side effects of light therapy.
[...]
> Authors' conclusions:
> Evidence on light therapy as preventive treatment for patients with a history of SAD is limited. Methodological limitations and the small sample size of the only available study have precluded review author conclusions on effects of light therapy for SAD. Given that comparative evidence for light therapy versus other preventive options is limited, the decision for or against initiating preventive treatment of SAD and the treatment selected should be strongly based on patient preferences. It has a plausible mechanism of action, and (so long as other treatment approaches aren't ignored) it's unlikely to do any harm.