CARMEL, Ind. -- Lisa Eppen has finally beaten the winter blues, a condition doctors call seasonal affective disorder, or SAD.

"I'm a complete victim of SAD. As soon as it turns winter, no sunshine, I get pretty sad," Eppen said.

Eppen's doctor has an explanation. During Indiana's gray and dreary winter, people's skin doesn't absorb enough sun to make enough vitamin D.

"It's required for mood, so perhaps some of our depression may indeed be related to vitamin D deficiency," said Dr. Eve Olson, director of the Olson Center for Wellness in Carmel.

A recent study published in the New England Journal of Medicine reported that 60 percent of Americans are vitamin D deficient.

The findings are particularly troubling to Olson.

"We've always known that vitamin D helps build strong bones, but it also boost the immune system and builds muscle strength," Olson said. "And we've learned it fights colon, breast, prostate and ovarian cancers. Patients that have the highest vitamin D levels have much less incidence of these cancers."

Like many Hoosiers, Eppen relied on milk to get her vitamin D.

"I thought I could at least get enough of it in my glass of milk," she said.

Although vitamin D is found naturally in milk and fatty fish, Olsen said the amounts consumed in those things aren't enough.

The government recommends getting at least 10 minutes of sun a day if you are fair-skinned and 20 minutes a day if you are dark-skinned. Olson suggests that if you can't get that much sun, take a vitamin D pill at least 1,000 units every day.

Those supplements are shedding new light on Eppen's winter blues.

"My mood level has stayed the same. It was really, definitely a difference."

Certain medicines may interact with vitamin D, so be sure to talk with your doctor before taking any supplement.