To know which vitamin you lack just have to pay attention to your body. Hair loss, trouble in sleeping, and even the sudden appearance of gray hair can be signs of lack of vitamins.
Vitamins are one of the essential nutrients of a healthy and balanced diet. Vitamin supplements should only be used by medical prescription, to compensate for the deficiency of these nutrients in very specific stages. Similarly, do not abuse vitamins, because an excess or hypervitaminosis may have counter-productive effects on health in the same way.