If you have bipolar disorder, you might notice your symptoms get worse as the seasons change. Some research says this may be related to sunlight.

Bipolar disorder is characterized by mood episodes. Often, the changes don’t seem related to anything happening in your life. But other times, they may be related to specific triggers, such as stress.

You might wonder whether the weather can affect your mood changes. Can sunlight prompt a manic episode? Can rainy, cold weather trigger a depressive phase?

It’s unclear what causes shifts in bipolar disorder phases (from mania or hypomania to depression or vice versa). But medications such as lithium can help lessen or prevent these changes.

The idea that changing seasons or weather might play a key role in causing a manic episode or hypomanic episode can be traced to a 1978 study.

This study examined hospital admissions for mania and found peak admissions for mania in summer and low admissions in winter.

Researchers also found a correlation between manic episodes and the following:

  • temperature in the month in question
  • mean length of the day
  • mean daily hours of sunshine

Since then, other researchers have examined the correlation between manic or hypomanic bipolar disorder phases and the year’s season.

A 2015 study examined 2,837 hospital admissions, finding that:

  • most admissions for mania were noted in the spring, summer, and midwinter months
  • people were more likely to be admitted to the hospital for mixed features in late spring and winter
  • bipolar disorder-related depression admissions were most likely in the spring and autumn months

Researchers noted a connection between bipolar disorder episodes and hours of sunlight in different months.

The 2015 study concluded that there appears to be seasonality, or seasonal patterns, in mood changes related to affective disorders, such as bipolar disorder.

These researchers weren’t alone in finding this correlation between sunshine and manic episodes.

A 2016 study supported a connection between sunshine and the manic phase of bipolar disorder. The large-scale study examined 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,” researchers wrote. “Higher admission rates were associated with more sunshine, more ultraviolet radiation, higher temperature and less snow, but were unassociated with rainfall.”

A 2008 study did not find an association between its 56 subjects and data such as hours of sunshine, temperatures, rainfall, etc.

Because of the study’s small size, researchers didn’t have enough manic episodes to track, so instead used other measures (a mania rating scale, for instance) to act as a stand-in for actual manic episodes. This makes the results of this study difficult to compare to other studies.

Bipolar disorder isn’t the only mental health condition that sunlight might affect.

Seasonal affective disorder, now known as major depressive disorder with seasonal patterns, is a form of depression that occurs during the winter months. Though both may cause worsening depression linked with changing seasons, this form of depression is different from bipolar disorder.

Some people also believe that the symptoms of borderline personality disorder can worsen due to the change of seasons. But there is little evidence that this is the case.

If your symptoms worsen during specific seasons, maintaining a routine and treatment plan can help.

Bipolar disorder treatments can help you maintain consistent moods throughout the year. Common treatments include psychotherapy (talk therapy), medications such as lithium, or a combination.

Though you might not be able to find sunshine all year round, there’s some evidence — including a 2020 review — that bright light therapy can help with depressive symptoms in bipolar disorder.

That said, the authors noted that they looked at only short-term effects, so they might have underestimated any negative effects of bright light therapy on mania or depression.

Bright light therapy is a common treatment for SAD. It uses bright lights to influence your circadian rhythm year-round, leading to better sleep and an improved mood.

It’s uncertain whether sunshine, rainfall, and temperature can prompt mood changes in bipolar disorder. Still, there is evidence that such changes may be related to or triggered by the weather.

The actual strength of these changes likely varies from person to person. The weather alone is unlikely to be the most important or sole cause of developing mania or hypomania. Still, it’s a prompt people with bipolar disorder should be aware of.