I'm sorry to hear that father is only awake for what sounds like a few hours a day. I agree with many of the other answers, as sleep disorders are
part and parcel of PD. That being said, there are a number of step that we take to try and help:
1) Many PD medications can actually cause sleepiness and we try to minimize the use of these. Specific "bad actors" in this regard include the dopamine agonists (mirapex and requip), Artane and sometimes even sinemet, although to a lesser degree usually.
2) Make sure that other medications aren't playing a role. A number of chronic pain meds can make people very sleepy, as can medications like ativan, valium or clonazepam.
3) We try to adjust patient's sleep cycle such that they sleep more at night and less in the day. We do this with a combination of "environmental modification" and medications depending on the patient. For example, we get the family to try and keep their PD patient up for almost all of the day if possible as well as try to make them get lots of sunlight and exercise (to whatever degree possible) during the day. Then at night we sometimes will use medications to make sure that they don't stay up most of the night and then sleep most of the day. The medication that we use depends a lot on the patient, whether they have dementia or not, etc.
4) Sometimes, depending on the situation, we can try to use medications like provigil or nuvigil to help patients be more alert during the daytime.
Indeed this is a very difficult problem and there are not easy solutions, but hopefully this provides a little guidance.