A new meta-analysis from the University of British Columbia takes a close look at the growing body of research into light therapy for patients with bipolar depression. Dr. Raymond Lam — a professor in the department of psychiatry, director of the Mood Disorders Centre at the Djavad Mowafaghian Centre for Brain Health, and research scientist at the Vancouver Coastal Health Research Institute - and his colleagues found that, when it comes to reducing symptoms of bipolar depression, light therapy holds promise.
Seven published trials of light therapy compared to a placebo in people with bipolar depression were found. When the results of the studies were statistically combined, overall, there was significant improvement in depressive symptoms with bright light compared to placebo conditions.
It was also that bright light was well tolerated in the studies, with few people having to stop the study for side effects or other reasons. Importantly, light therapy did not cause a switch from depression to mania, as can happen with antidepressants in people with bipolar disorder.
Circadian Rhythms Affected
It is unknown exactly how light therapy works for seasonal and nonseasonal depression, and whether it is the same mechanism for bipolar depression.
What we do know is that light acts through the eyes to regulate the biological clock located in a tiny region of the brain, known as the suprachiasmatic nucleus. We also know that there are disturbances in the biological clock that affect circadian rhythms in people with bipolar disorder, including hormonal rhythms, sleep, cognition, and other behaviors.
However, we don’t know if regulating circadian rhythms is the reason why light therapy works as an antidepressant.
Other studies have shown that light can affect the major neurotransmitters, or chemical messengers, involved in mood and behavior, such as serotonin and dopamine. So, light therapy may work similarly to antidepressants by directly acting on these neurotransmitters.
Better Treatment Needed
There are fewer available treatments for bipolar depression compared to other types of depression, so it is important to find new effective treatments. Antidepressant medications are not clearly effective and they may worsen mood cycling in people with bipolar disorder.
Also, many people with bipolar disorder must take other medications such as mood stabilizers. That’s why it is important to determine if a non-medication treatment, like light therapy, is effective, because it can be used without worrying about drug interactions.
Although positive results were found, the work also found limitations in the previous studies. For example, only small numbers of people were studied, there were many different types and “doses” of light therapy used, the follow-up periods were very short, and most patients were taking other medications for bipolar disorder.
Moving forward, more research with larger samples of patients is needed to conclusively show that light therapy is effective for bipolar depression. We also need to determine the optimal conditions for light therapy, including what wavelength of light works best, how bright the light needs to be, and how long patients should use the light.
Since bipolar disorder is a chronic condition, scientists also need to further examine how light should be used for maintenance treatment to prevent future depressive episodes.
 Lam, R. W., Teng, M. Y., Jung, Y.-E., Evans, V. C., Gottlieb, J. F., Chakrabarty, T., … Sit, D. K. (2019). Light Therapy for Patients With Bipolar Depression: Systematic Review and Meta-Analysis of Randomized Controlled Trials. The Canadian Journal of Psychiatry. https://doi.org/10.1177/0706743719892471