Some people with bipolar disorder can track their moods by looking at the calendar: If winter is coming, they can expect to have seasonal depression. If it’s spring, they might experience mania or hypomania.

“This seasonal component is related to the amount of sunlight,” says Melvin McInnis, MD, director of psychiatry programs at the University of Michigan Depression Center in Ann Arbor. “In the wintertime, people are more depressed. The patient says, ‘Every January, I start to go down.’ Then in the spring, the amount of sunlight destabilizes their mood to the point of them becoming manic and hypomanic. We [doctors] often refer to the ‘manic month of May.’”

Doctors have long distinguished between seasonal depression and seasonal bipolar disorder. Seasonal depression — commonly referred to as SAD, for seasonal affective disorder — is a mood disorder brought on by the biological effects of a lack of sunlight. Typically experienced in the late fall and winter, it is particularly prevalent in northern regions, according to the American Psychological Association (APA). What distinguishes seasonal bipolar disorder from SAD is the presence of a manic episode within a given period of time.

People must have a history of manic or hypomanic episodes (the extreme highs) to be diagnosed with a bipolar mood disorder, explains Ken Duckworth, MD, medical director for the National Alliance on Mental Illness (NAMI) and an assistant professor at the Harvard University Medical School. If that’s not part of their medical history, he says, then their seasonal winter response is a depressive disorder and not bipolar.

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Warning signs of seasonal affective disorder and an upcoming bout of depression can include feeling hopeless, tense, or stressed, and losing interest in people or activities you normally enjoy, according to Harvard University. You may notice this happens to you every year in late fall and early winter and doesn’t get better until late into the spring, Dr. Duckworth says.

Seasonal Shifts in Mood: Who Is Affected?

Why do some people experience a seasonal component to their bipolar disorder and others don’t? One of the most prominent developing theories has to do with circadian rhythms — the “biological clock” — the body’s internal, rhythmic response to changes in a 24-hour day, especially sunlight. This response is controlled by a complex set of genes commonly referred to as “clock genes.” If some of these genes are abnormal, you may have a higher risk of developing seasonal bipolar disorder, according to a study published in August 2015 in Frontiers in Psychology.

But while it's likely that in some people there's a bipolar-SAD connection, it hasn't yet been proven. Duckworth says, “There’s nothing definitive.”

Gabe Howard, 38, of Columbus, Ohio, was diagnosed with bipolar disorder in 2003 and he also experiences SAD when it’s dark, cold, and snowy outside. “I definitely feel more sad in the winter,” he says. “I’m sleepier, and I’m more tired, and I can be more irritable.”

Howard has to work a little harder to stay on an even keel during the winter months. When he was first diagnosed with bipolar disorder and SAD, his doctor needed to adjust his medications for the seasons, but he hasn’t had to do that for years. Howard believes adding therapy skills — including radical acceptance, where you learn to accept what is and not turn things into catastrophes — to his treatment regimen helps him cope better with both illnesses. Also, during the winter he’s especially mindful of eating healthy and participating in activities he enjoys, such as watching hockey on TV. He also tries to sleep more, but not too much. “I find I need [to bank] more reserves in the winter,” he says.

7 Treatments for Bipolar Disorder and Seasonal Depression

If you and your doctor determine that your bipolar disorder has a seasonal component, the doctor may suggest various treatment options, particularly related to depressive episodes, which tend to be more severe among those with seasonal bipolar disorder.

1. Exercise. If the depression is mild, you may not need additional medication. Rather, you may be able to manage your depression with healthy lifestyle habits, including exercise. Take a brisk walk during the sunniest part of the day, the APA suggests. Even short bouts of exercise can help lift your mood.

2. Maintain a routine. Keep your daily living patterns — waking, sleeping, eating — the same as much as possible, Duckworth says. “You can create a rhythm where your body will be well regulated.”

3. Light therapy. Light therapy, such as special light boxes used 30 minutes a day, can be helpful. Light therapy may work faster than medications in some people, according to NAMI. But light therapy must be used carefully. Light therapy has the potential of inducing a manic reaction in the person with bipolar disorder, Duckworth says. Too much bright light “tends to correlate with mania just as too little correlates with depression,” he explains.

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Bright-light therapy should be seen as a regular antidepressant treatment in people who have seasonal bipolar depressive episodes, according to a study published in October 2015 in the journal L’Encephale. But it's not a guaranteed treatment. Howard, for instance, found it didn’t work for him.

4. Melatonin. If the depression is severe, light therapy may be prescribed in combination with medications, such as melatonin. A naturally occurring chemical transmitter that helps regulate circadian rhythms, melatonin has long been used in synthetic form to reset the biological clock for people suffering from jet lag or insomnia. But over-the-counter melatonin may not be enough. A study published in 2012 in the journal Expert Opinion on Investigational Drugs suggests that antidepressants that regulate melatonin may be the best treatment for depressive disorders, including SAD and bipolar.

5. Stress reduction. Stress is a major contributing factor in SAD and can trigger bipolar episodes, NAMI says. It’s hard to avoid all sources of stress, but try to limit those you can. If you have seasonal bipolar disorder, don’t change jobs or your home in the dead of winter, Duckworth advises. Meditation and mindfulness also can help you manage stress, according to the APA.

6. Vitamin D. Low levels of vitamin D have been linked to depressive disorders. Researchers in Australia reported in 2014 in the journal Nutrients that for some people with a vitamin D deficiency, supplementation can work as well as antidepressants in managing depression. Howard takes vitamins, including D, year-round and finds this helps stabilize his moods. Ask your doctor to test your vitamin D levels and whether you should take supplements.

7. Plan ahead. Be proactive about your bipolar and SAD disorders, Duckworth says. Learn about your seasonal patterns of depression and know what can trigger them so that you can try to avoid them, especially in winter. Having a plan to deal with potential stressors can help, too. Know what you’ll do if a snowstorm leaves you homebound, for example. Knowing you have solutions will make it easier to deal with problems should they arise, states the bipolar caregivers guide of the University of Melbourne.

The good news about seasonal bipolar disorder relates to the management of your symptoms. Unlike people with bipolar disorder without a seasonal component, you can anticipate the coming of your mood shift simply by watching the calendar. You and your care team can be prepared with a solid treatment plan in hand.

Additional reporting by Beth W. Orenstein.