It really depends on what you are trying to measure in what time frame. If you are concerned about your overall daily mental performance re: coding or the sort

It really depends on what you are trying to measure in what time frame. If you are concerned about your overall daily mental performance re: coding or the sort that would require focused thinking, one indicator to note could be to see how long it would take you to do things that you normally expect to get done in a reasonable manner. It's really a matter of tracking yourself against your history. If you've had a log of how much it took you to do something, you could compare your activities against that for starters. Other thing to note would be the level of distraction you may be under. A good way to track this is to do it indirectly using something like RescueTime. If you find that the app you should be using is very low or you see a spur of new, bite sized activities popping up, then maybe your performance is taking a hit from tiredness. But really, I would just worry about if things are getting done within the time frame you expect + compare against your control (past).