Feeling tired all the time is a drag, isn't it? You get a full night's sleep, but you still feel like you could nap at your desk. You might chalk it up to stress or a busy schedule, but it could be something more: a vitamin D deficiency. This is a much more widespread issue than people realize. It is estimated that over a billion people worldwide have low vitamin D levels and may not even know it. Understanding what a vitamin D deficiency is and what it does to your body is t