"Keep your face to the sunshine, and you cannot see the shadows," Helen Keller once said. And while Keller's words might oversimplify the matter, you do need sunlight to maintain optimum health and wellness. Sunlight can influence brain health, sleep, immunity, healing, and even intimate desire.
The health benefits of sunlight mostly come from vitamin D, a crucial element human beings soak in during sun exposure. Without enough vitamin D, the body cannot operate at its best. According to the National Institutes of Health, those at highest risk of vitamin D deficiency include breastfed infants (human milk alone contains an insufficient amount of vitamin D), older adults, people with darker skin - and anyone with limited exposure to the sun.
If you're (literally) in the dark, your body might exhibit less-than-sunny symptoms.