I guess some one using a Power Tap could try and track how they are working for several days at a time. But for my own experience, I would have to say there simply is no way to measure performance this finely.

For the most part, like the weather, you will find an ebb and flow to your power range. These sorts of statistics are charted by the "pro" coaches all the time and they would tell you there's no good way to account for why certain riders will reach certain "zones" after multiple days of racing....... (all though they surely would like to know the why and how of it)