How harmful is too little daylight or sunlight?

Sunlight is by far the most important source of vitamin D. Food contains hardly any of it naturally, and fortified products rarely make up the shortfall adequately. As a result, more than half the world's population is at risk of deficiency.

The strongest evidence concerns bones. In children, vitamin D deficiency leads to rickets: bones become deformed because they mineralise poorly. In adults and older people, low levels cause weaker bones, bone loss (osteoporosis) and a higher risk of fractures. Vitamin D3 at 700 to 800 IU per day, combined with calcium, modestly but measurably reduces the risk of falls and fractures in older people. This effect is clearest in elderly residents of care homes; for other groups the evidence is less certain.

Several studies link low sunlight exposure to a higher risk of conditions such as colorectal cancer, cardiovascular disease and multiple sclerosis. One review estimated that insufficient sunlight may be responsible for around 340,000 deaths per year in the United States and 480,000 in Europe^{1,2,3,4,5,6,7}. However, these are model-based estimates derived from statistical associations, not proven causes.

What stands out is that vitamin D supplements have so far not convincingly prevented those chronic diseases. This suggests that sunlight works through more than vitamin D alone. For instance, emerging research shows that UV radiation releases nitric oxide from the skin, which may lower blood pressure. But this is still at an early stage of investigation and has not yet been confirmed in large human studies.

For most people, 5 to 30 minutes of daylight per day on the arms, legs or face is sufficient for adequate vitamin D production, depending on skin type and the strength of the sun. Normal use of sunscreen does not interfere with this. Excessive exposure should be avoided in any case, as it increases the risk of skin cancer.