I'm nearly 2 decades older than u, Kim. And, I rarely get tired. Plus, I seem to get by ok with less sleep lately. However, I notice a number of my friends seem to fade easily these days.
I don't think you're the type that gets tired often, but as Josie said, burnt out maybe? U can't burn the candle at both ends forever. On the plus side, the holidays may give u a chance to re load.
Then again, maybe not getting enuff Kim time is your problem?