How harmful is too little daylight or sunlight?

Sunlight is by far the most important source of vitamin D for humans. Food contains virtually no vitamin D naturally, and fortified products are often insufficient. More than half of the world's population is at risk of deficiency, purely from too little sun exposure. This makes it a global public health problem.

The most directly proven harm from too little sun runs through vitamin D deficiency. In children, this leads to rickets, a condition in which bones become deformed because they do not mineralise properly. In adults and older people, low vitamin D levels cause elevated parathyroid hormone (PTH), lower bone mineral density, and a greater risk of bone loss (osteoporosis) and fractures. The evidence for this is reasonably strong.

Regarding supplements: vitamin D3 (700-800 IU per day) combined with calcium produces a modest improvement in bone density and reduces the risk of falls and fractures in older people. This effect has been most clearly demonstrated in institutionalised older women. For other groups the benefit is less certain.

Multiple studies link low sunlight exposure to an increased risk of conditions such as breast cancer, colorectal cancer, cardiovascular disease, multiple sclerosis, type 1 diabetes, and infectious diseases. One review even estimated that insufficient sunlight exposure may be responsible for approximately 340,000 deaths per year in the United States and 480,000 in Europe. These are, however, model estimates based on statistical associations, not proven causal relationships. Caution in interpretation is warranted.

Notably, oral vitamin D supplementation has not convincingly been shown to prevent these chronic diseases. This suggests that the benefits of sunlight do not run exclusively through vitamin D. Newer research points to other mechanisms, such as the release of nitric oxide (NO) from the skin by UV radiation, which may lower blood pressure. This evidence is still preliminary.

For most people, 5 to 30 minutes of sunlight 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 broad-spectrum sunscreen does not impair vitamin D production in healthy people. An exception applies to people with a photosensitivity disorder who also use protective clothing and shade. Furthermore, excessive exposure must in any case be avoided because of the risk of skin cancer, as is sharply illustrated in people with albinism, who face greatly increased risks of skin cancer and premature death with excessive exposure.