IF YOU’RE LIKE most working Americans, you more or less drag yourself out of bed in the dark, trudge your way to work, then sit in a fluorescent-lit office space for 10 hours, after which you make the same grudging trek back home… in the dark.

Sure, this is a morose depiction of day-to-day life—but in winter, this isn’t a far cry from the truth.

Problem is, all this time in the dark, or at least away from natural sunlight, is causing many of us to become deficient in vitamin D. And, regardless of whether we’re in a balmy spring, scorching summer, or the dreary, chilly months that stretch from fall to winter, our basic needs don’t change.

We desperately need the sunshine vitamin. Here are the 7 most important reasons why.

Talk to your doctor about taking a supplement. You want up to but no more than 4,000 IU a day, per the Institute of Medicine.