When the winter blues strike, many people blame the shorter days and longer nights. It’s a clinical condition known as seasonal affective disorder or SAD for short.

Symptoms including persistent low mood, irritability and feeling stressed or anxious, can pounce without warning, said Dr. Norman Rosenthal, clinical professor of psychiatry at Georgetown University School of Medicine.

“[People] become slowed down and need more sleep,” he said. “They withdraw from friends and family and relationships and work can suffer.”

About 15% of the U.S. population suffers from SAD. Of that number, roughly a third experience more severe symptoms.

Rosenthal said there are four simple ways to combat the condition.

“First and foremost, get more light,” he said. “This can be done with light fixtures. It has to be big enough. And you have to do it regularly.”

Taking walks outside is another option to get more light therapy. Brisk walks can also add more exercise to a daily routine, which helps alleviate symptoms, he said.

Dr. Rosenthal has studied the condition in patients for nearly 40 years, leading a team of researchers back then that first described the condition.

Another way to ease SAD: regular social engagement.

“Keep your friendships and social dates, whether they’re lunch dates or whatever,” Rosenthal advised. “Don’t veg out because if you behave like a hibernating bear, you’ll feel like a hibernating bear.”

Finally, challenge negative thinking.

“Don’t say, ‘I can’t do this’ or ‘I’ll never be any good,” he said. “Understand this is part of a condition. It has very predictable qualities and there are things you can do about it.”

If simple measures to ease symptoms don’t work, Rosenthal said sufferers may need to consider antidepressants.

“I prefer to try to treat without medicines wherever possible,” he said. “But it is much better to use a medicine and function well and feel well than to avoid it and have a struggle on your hands.”