By default, a session lasts until there's 30 minutes of inactivity, but you can adjust this length so a session lasts a few seconds or several hours. A session can be as short as a few seconds or as long as several hours. When a user, say Bob, arrives on your site, Analytics starts counting from that moment. If 30 minutes pass without any kind of interaction from Bob, the session ends. However, every time Bob interacts with an element (like an event, social interaction, or opens a new page), Google Analytics resets the expiration time by adding on an additional 30 minutes from the time of that interaction.
Another reason for this is that sunlight is all that is required for the synthesis of vitamin D. However, in those who do not get enough of these sources of the vitamin, rickets may develop as a sign of the deficiency. Other signs of Vitamin D deficiency include a reduction in calcium absorption and hair loss. Vitamin D deficiency causes hair loss through a disruption of the hair growth circle. The deficiency shortens the anagen phase and prolongs or speeds the onset of telogen and catagen phases.
This is why vitamin D deficiency and hair loss go together. Studies have shown that inhibiting the vitamin D receptors in the follicle can actually cause alopecia (hair loss). If you were to deprive your body totally of vitamin D, you would not have one of the necessary building blocks essential to hair growth.