It's a condition similar to depression that occurs each winter as sunlight levels decrease then improves in the spring.

The good news is sad is highly treatable, says Dr. Paul Dean, the director of the Winter Depression Clinic at the Yale School of Medicine.

Here are seven things to know about treating the disorder.

One, the standard method for treating seasonal affective disorder is light therapy, using a lightbox device.

The research best supports a half an hour of 10,000 locks of light, seven days a week before 8 a.m.

10,000 lux is a measure of brightness of light. That's about the brightness of being outside in July.

The research suggests that light is most potent applied in the morning.

That simple environmental therapy, that is is a very powerful and fully effective treatment for most people with seasonal affective disorder.

Two, where the lightbox is placed is key.

The light has to be visual in the visual field, but we don't recommend just staring at it.

You probably want it to be a little bit elevated.

They've got to be able to set up a device on the counter or table top, eat breakfast, read the paper, whatever they do in the morning.

Three, light therapy routines may need to be adjusted depending on the patient.

When people start doing light therapy, some people may suffer a bit of headache or glare and we have advise them just to do the treatment for a slightly shorter period of time and usually they get used to it.

It may be that a longer period of exposure to a lower level of light might be effective.

Some people may get a little wired or over-activated.

And once again, we recommend they just shorten the treatment briefly.

Now people with bipolar disorder, you can trigger elevated mood or symptoms of elevated mood.

So anyone with bipolar disorder should be getting light treatment in collaboration with a qualified clinician.

Four, medication can also be used to treat seasonal affective disorder.

The only antidepressant that's actually approved by the FDA is Wellbutrin for the prevention of seasonal affective disorder.

That indication proved by the government is to start taking Wellbutrin before the winter begins.

But other antidepressants do seem to work.

Five, even though vitamin D production is tied to sunlight exposure, it's not clear if taking vitamin D supplements helps treat sad.

We don't think that vitamin D deficiency has anything to do with seasonal affective disorder in the vast majority of patients.

There may be a few patients who have a true clinical deficiency.

The one trial giving people vitamin D for seasonal affective disorder found no therapeutic effect.

Six, simple behavior changes can combat sad and other forms of depression.

Well, there's some very interesting research that cognitive behavioral therapy that gets people up and going in the winter time can be effective for seasonal affective disorder.

And indeed, if you get people up and going, they're going to feel better, you're also going to be exposed to more light.

It's a cascade.

So we encourage people to do all the things that sustain them.

Seven, light therapy can help even if your seasonal symptoms are not severe enough to be diagnosed as sad.

They just don't feel as vigorous during the winter, their energy is worse.

Their mood isn't quite as good.

All of these winter symptoms can also be treated with bright light.

So even if people don't have the full syndrome of seasonal affective disorder, they may benefit from light treatment.

If you suspect you may have sad or another mental health disorder, check with a healthcare professional about how you're feeling and possible treatments.

For the PBS News Hour, I'm Casey Coon.