Actually what's more likely is that it's... off-cycle(?) for me. I'm not sure if it's accurate but I know my wife often mentioned that people have a certain cycle, and timing your sleep to coincide will tend to make you feel much better. As in, something like this: her sleep schedule I believe was on a 4-hour cycle, so she either needed to get 4 hours or 8 hours (or 12 hours) of sleep to feel well rested. I have no idea if it's true, but I have noticed that sometimes (often) getting less sleep seems to make me feel better. However, if I sleep for a LONG time (somewhere in the 10 or 12 hour range) I also feel refreshed and well-rested. Maybe I have a 5-6 hour cycle and so I need ~5 or ~10 (or maybe ~6 and ~12) hours of sleep to feel right.
I dunno. If anyone actually does know something about sleep patterns and cycles, please, feel more than free to comment, as I'd be happy to learn something about it (without having to put in the effort to research it myself ;) ).