I should get 8 hours but I rarely get more than 7 - I'm really lousy at going to bed (which is why I hate getting up at 7am grrr).

I think with afternoon naps it's largely a question of time. If you oversleep generally you often feel worse than when you don't sleep enough and I think this is the case with afternoon naps. An hour long sleep is the optimum in the afternoon I think - any more or less and you will feel worse than when you started.