People with bipolar disorder who were exposed to bright white light at midday experienced a significant decrease in symptoms of depression, found a study conducted at Northwestern University Feinberg School of Medicine.

Bipolar disorder causes manic episodes of elevated mood, energy and cognition, followed by major depressive episodes of lowered mood, energy and cognition. It affects up to 4.4 percent of the population.

Previous studies found that morning bright light therapy reduced symptoms of depression in patients with Seasonal Affective Disorder (SAD). But the treatment caused side effects in some bipolar patients, including mania. The new study used a new midday light therapy in an effort to provide relief and avoid side effects.

Study participants had at least moderate depression as well as bipolar disorder, and were on a mood stabilizing medication. They were randomly assigned to either a 7,000 lux bright white light or a 50 lux placebo light.

They were instructed to place the light box about one foot from their face for 15-minute sessions to start. Every week, they increased their exposure to the light therapy by 15-minute increments until they reached a dose of 60 minutes per day or experienced a significant change in their mood.

More than 68 percent of patients who received bright light therapy between noon and 2:30 p.m. for six weeks experienced normal mood levels compared to 22.2 percent of patients who received a placebo treatment of dim light.

The group receiving bright light therapy also had a much lower average depression score of 9.2 compared to 14.9 for the placebo group. Their level of function was also significantly higher, meaning they could go back to work or complete chores around the house they hadn't been able to finish before being treated.

"Effective treatments for bipolar depression are very limited," said lead author Dr. Dorothy Sit. "This gives us a new treatment option for bipolar patients that we know gets us a robust response within four to six weeks."

Patients also experienced few side effects from the therapy, and none experienced mania or hypomania, a condition that includes a period of euphoria, agitation, rapid speech, racing thoughts, and risk-taking behaviors.

"As clinicians, we need to find treatments that avoid these side effects and allow for a nice, stable response. Treatment with bright light at midday can provide this," said Sit.

The study was published in the American Journal of Psychiatry.