Thank you all so much. These are great suggestions and I have learned a lot in the last few hours.
It seems that in this case, the simplest answer is the best one...
John Fruits suggested that cutting the power through a timer or photocell would be the best solution. My concern was that the lights would not come back on to the pre-set program. I was able to test that this evening and as Len Zenith Jr said, they do in fact come back on the same way they were turned off. So, a basic (and inexpensive) photocell control looks like the best way to go in this situation. Simple and effective.
Thank you all again so much for your help.