Suggest Treatment For Lethargy And Vitamin D Deficiency

I have a serious problem with my husband. He is constantly tired. He s had blood work done and the first thing they said was it was a Vitamin D deficiency. He took the pills, his D levels came back up but he is still tired all the time. We had him tested for Diabetes and the tests came back negative. He doesn t drink a lot of caffeine and we have changed our eating habits to more fresh vegetables, fruit and lean protein. We exercise regularly but he is still tired all the time. He only sleeps for an average of 6 hours a night and needs to take naps during the day and still goes to bed before I do. Any ideas on what could be going on that the doctors we are seeing are not seeing? I m desperate for answers. We want to have a baby but he can t even stay awake for that. Please help. Thank you.