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.

This is somewhat limited information to go on.  Sundowner's usually presents with other symptoms, including the agitation.  Being that the problem starts at sundown and the agitation occurs then this is very likely, but look at whether or not other symptoms are also present such as confusion, pacing, rocking, mood changes, etc.

It is not likely a vitamin D deficiency.  But you can get him out in to the sun more for the best source of vitamin D.

Sundowners is a syndrome, not a disease.  Syndromes are groups of symptoms that can have multiple causes.  It is often associated with dementia though.  Therefore, If you suspect sundowners then I recommend focusing on the brain and the rest of the nervous system with supplements such as lecithin, EPA/DHA and B₁₂.  You can also try giving him glycine when he gets agitated to help keep him more calm.