The important role of vitamin D in human body:

 Vitamin D is made under the skin, and it is metabolized in the liver and kidneys, where it is activated. It helps the human body to absorb Calcium and phosphorus from the intestines and precipitate them in the bones. It plays a vital role in the production of insulin in the pancreas and in the growth of the cells of the spinal cord. It also plays an important role in the growth of the cells of the upper layer of skin. This is well known to doctors and medical scientists; but the new thing is that the best time to benefit from sunlight is at daybreak and sunset. That is why the Dawn Prayer plays a very vital role in man’s good health. In other words, those who are not exposed to sunlight and sleep for long hours suffer from osteomalacia (softening of the bones).
In addition to its being a kind of worship, Allah’s Divine Command is undeniably abundant in benefits to man’s good health. This can be inferred from Allah’s Words,

“And you might have seen the Sun, when it rose, declining to the right from their Cave, and when it set, turning away from them to the left, while they lay in the midst of the Cave. That is one of Allah’s Signs.”

[XVIII; 17]

 Those who get up before sunrise and before sunset get the benefit of ultra-violet rays. Scientists say that it is not important to be directly exposed to sunlight, for the diffused light at sunrise and sunset help the skin make vitamin D, which contributes to the precipitation of Calcium in the bones and strengthening them. It also helps activate the pancreas and increases growth of the bulb of bones and skin cells.