Sundowners or lack of vit. D

My 83 yr old father is having problems sleeping. He can't go to sleep, or stay asleep. He starts feeling anxious when it starts getting dark. It's gotten to the point where he is sleeping at friends house, or is having them spend the night. Then he can sleep better. His doc gave him some sleeping pills but they only worked one night. He also complains of excessive night sweating. Could this be sundowners or caused by lack of vit D? I asked his doc to check his D level. She said "If you're worried, give him some." But it's hard to get him to take more pills.