Egg or chicken?

Today, for some reason, I fell asleep in the afternoon. I haven't done that for a very long time. So, the question is whether it's simply that I was very tired - thanks to the dogs getting me up at 05:29 - or whether it might be a residual side effect of last year's radiation treatment. I'm inclined towards the former interpretation, but that brings me to a question I've been asking recently when I feel tired. Do I feel tired because I'm getting older and get tired more quickly? Or do I feel tired because I feel better and I'm able to do more than previously, thus getting more tired? It's a tough one - which comes first? Chicken or egg? Maybe I should keep a diary!