Dr. Mercola has written an interesting article detailing the connections among vitamin D deficiency, B vitamin deficiency, gut health, and sleep cycles.  He summarizes research that dates back to the 1980s but has not been broadly publicized.  I think it’s a level of complexity and detail that’s easy to read, but a bit too much to try to summarize on one paragraph, so if you or someone you know struggles with sleep, I suggest reading what he has to say here.