What's you opinion on vitamin D?

Elise stopped taken it when she was 1 years old. Now I'm finding info that says it can help with sleep? Is there other benefits of taken vitamin D besides that is used due lack of sufficient sunlight?

    I give the kids D3 in the winter. But stop in the summer because they should be outside enough to get it from the sun.
    It's one of those that as long as you keep the right level in your body, it greatly strengthens your immune system.