People with bipolar disorder often suffer changes in mood that can be unrelated to anything going on in their lives. Research has shown that sometimes, however, a change in bipolar phases may be related to certain triggers, stress being a primary one for many.

But what about the weather? Can sunshine trigger a change in a person’s bipolar manic phase? Can rainy or cold weather trigger a depression phase?

To date, it is not yet clear what causes the change in a person’s bipolar disorder, switching from mania to depression or vice-a-versa. It is known that medications such as lithium can help attenuate or prevent these changes from occurring altogether.

Bipolar & Sunshine: Is It Seasonal?

The idea that changes in the seasons or weather might play an important role in inducing a manic or hypomanic episode in bipolar disorder can be traced back to Myers & Davies’ study from 1978 that examined hospital admissions due to mania and found a peak of mania episodes in summer and a nadir in winter. These same researchers also found a correlation between mania episodes and temperature in the month in question as well as the mean length of the day and mean daily hours of sunshine in the month before.

Some researchers have examined the correlation between a person with bipolar disorder’s change into a manic or hypomanic phase and the season of the year. Dominiak et al. (2015), for instance, found in their study of 2,837 hospital admissions, most mania admissions were noted in the spring and summer months, as well as in midwinter. These same researchers found that in late spring and winter a person was more likely to be admitted to the hospital for a mixed episode. And depression episodes were most likely to be seen in the spring and autumn months.

They went on to conclude:

The association between frequency of admissions and monthly hours of sunshine was observed in some age and sex subgroups of patients with bipolar disorder and single depressive episode.

The results support the seasonality of admissions of patients with affective disorders

These researchers weren’t alone in finding this correlation between sunshine and bipolar disorder’s manic phase. Newer researcher from Medici et al. (2016) also found evidence to support a connection between sunshine and the manic phase of bipolar disorder. Their large-scale study examined a whopping 24,313 hospital admissions of people with mania in Denmark from 1995 to 2012.

“There was a seasonal pattern with admission rates peaking in summer,” the researchers wrote. “Higher admission rates were associated with more sunshine, more ultraviolet radiation, higher temperature and less snow, but were unassociated with rainfall.”

Korean researchers Lee et al. (2002) found a similar correlation in 152 patients with bipolar disorder who were admitted into two hospitals in Seoul, South Korea: “The mean monthly hours of sunshine and sunlight radiation correlated significantly with manic episodes.”

A flawed 2008 study (Christensen et al.) couldn’t find an association between their 56 subjects and climate data (such as hours of sunshine, temperatures, rainfall, etc.). But the study’s small size meant they really didn’t have enough manic episodes to track, and so the researchers ended up using other measures (a mania rating scale, for instance) to act as a stand-in for actual mania. This makes the results of this study difficult to compare to other studies.

Does the Weather Cause Mania in Bipolar Disorder?

While it’s unclear whether or not weather — climate factors such as sunshine, rainfall, and temperature — actually cause mood changes in bipolar disorder, there appears to be strong, replicated scientific evidence that such changes may be related to or possibly triggered by the weather.

The actual strength of these changes likely varies from person to person. Weather alone is unlikely to be the most important or sole cause of a person developing mania or hypomania — but it seems like that it can be a trigger that people with bipolar disorder should be aware of.