It’s that time of year, when twinkle lights make houses and trees sparkle, smiling snowmen stand proud on front lawns and everyone salesperson and random stranger wishes you a happy holiday — and yet, you feel anything but.

For many, it’s a funk that’ll pass, and for many people it is; for others, the mood shift runs a little deeper: Seasonal affective disorder (SAD) is a type of depression most often linked to winter. About 5% of Americans suffer from it; an additional 10% to 20% experience a milder form. The fact it gets so dark outside so early plays a role. Here’s what else you need to know:

The symptoms:

Symptoms include hopelessness, irritability and fatigue; hunger, especially for sweet or starchy foods, and subsequent weight gain; oversleeping; having a tough time concentrating; and/or wanting to avoid social situations. In most cases, symptoms appear in late fall or early winter, and fade in the spring.

The treatments:

Light therapy seems to work for most people. You might sit in front of a light box to expose you to about 30 minutes of bright light every day, which appears to affect mood-related chemicals in your brain, eases symptoms. Some people with SAD need antidepressants, others benefit from talk therapy as well. What doesn’t work: Tanning beds — the lights are high in ultraviolet rays, which harm both your eyes and your skin. Talk to your doctor if you think you have SAD.

Can I prevent it?

No, there’s no known way to stop seasonal depression from developing, but you can take steps to manage symptoms early on, and keep them from getting worse. Spend some time outside, even when it’s cloudy; natural light helps, especially within two hours of getting up in the morning. Eat well — a healthy diet boosts energy; exercise regularly to help relieve stress and anxiety; and hang out with your friends often, which can provide tremendous support during winter months.