Christmas Light Theory

My dad came up with a theory about why my Christmas lights only work for a year, then are dead when I try to use them again. Perhaps they will only light for a certain number of hours. Since I have them on from dusk to dawn from the day after Thanksgiving to sometime after January 6th, it is a possibility. I work my lights hard. But still, you'd think they'd last a bit longer. Anyone else have any ideas?