Almost 40 million people in the U.S. have been diagnosed and treated for migraine. That adds up to more than 160 million workdays lost every year — not to mention the severe pain and misery the headaches cause.

Migraine triggers vary from person to person. Stress, hormone shifts, irregular eating patterns, sleep disturbances, alcohol, smoking, and even pickled foods may set one off.

But a new study has identified a universal trigger that may come as a surprise: vitamin D deficiency.

It seems that a deficiency of vitamin D may allow increased inflammation, especially neurovascular inflammation in the brain that affects the way the endothelial cells lining blood vessels regulate blood flow.

And that’s not confined to any demographic. Vitamin D deficiency is related to migraine for all ages, genders, races, and incomes.

If you're a migraine sufferer, get a blood test to check your vitamin D level. If it’s low, take 1,000 IU of D3 daily, or more if recommended.

And focus on foods that contain vitamin D, such as mushrooms, salmon, sardines, and fortified tofu and cereals (with no added sugar).

Exposure to sunlight is also important.

In most of the U.S., spending 5-30 minutes in the midday sun — without sunscreen — between February and November three or more times per week is enough to maintain healthy levels among light-skinned people. For darker skin, it takes more time; one study recommends dark-skinned men opt for 90 minutes of sun exposure three times a week.