As summer approaches, it is important to be careful about too much sun exposure. But we all need some sunlight in our lives.

For most people, being outside on a sunny day is a warming experience inside and out. It's hard to be gloomy on a bright day, and there's good evidence that our moods tend to cycle with the seasons. This is especially so for people with depression or seasonal affective disorder (SAD), but is true to some extent for everybody.

Equally important, if not more so, is vitamin D, which comes from two sources: our diet (primarily fatty fish and fortified products such as milk and cereals) and sun exposure. Our skin cells manufacture vitamin D when exposed to UV light from the sun. 

This vitamin is well known to be critical for bone health. Low levels increase the risk of osteoporosis, and can cause bone pain and muscle weakness (in children, vitamin D deficiency is the cause of rickets). When vitamin D deficiency is corrected, patients often report reduced overall pain and sometimes even improved mood and sense of well-being. There is some evidence that vitamin D may also help to prevent certain types of cancer (especially colon), but more research is needed to confirm this claim.

Vitamin D deficiency is surprisingly common in adults, especially beyond age 50. The most reliable form to measure in a blood sample is 25-hydroxy vitamin D. A value of 20 ng/ml used to be considered the lower limit of normal, but recent data suggests that 35-40 ng/ml is a better minimum target for most people. One of the most significant risk factors for vitamin D deficiency is inadequate sun exposure. Causes of poor sun exposure can include living farther from the equator, increased cloud cover or pollution, darker skin pigment, and constant use of sunscreen. In addition, individuals who are unable to get outside very often are at increased risk.

So how much sun do you need? It depends in part upon the climate in which you live, but most people reach maximum vitamin D production after 5-15 minutes of daily unprotected (i.e. no sunscreen) exposure to the hands, face or arms. Longer unprotected exposure on a regular basis increases the risk of skin damage. But with adequate protection, more is probably better for improving your mood.

If your sun exposure is inadequate or carries too much risk, you should change your diet or take a vitamin D supplement. The recommended daily intake is typically 200-1000 units.

Don't go overboard with the supplements. Excess vitamin D can also cause health problems, but not unless you exceed 2000 units/day for a prolonged period. People with vitamin D deficiency typically need to take much higher doses for a few months in order to restore their body's reserves, and usually don't need to worry about overdosage. The National Institutes of Health has a useful fact sheet about vitamin D.