USEFUL: How To Sleep To Win. This bit on napping is interesting:
In the book, Dr. Maas states that if you are getting an adequate amount of sleep at night you may not need to nap in the afternoon. However, if you need or want to nap in the afternoon he makes two recommendations: nap for 20 minutes or nap for 90 minutes.
His rationale is that the shorter nap (20 minutes) takes you through the first two stages of the sleep cycle (excluding the REM stage), while the longer nap (90 minutes) takes you through the entire five stage sleep cycle (including the REM stage). As a result, in both cases you are not waking up while still in REM sleep.
You know that crappy feeling you get sometimes when you wake up from a nap in the afternoon?
That’s a result of you waking up in the middle of the REM stage of the sleep cycle.
So if you truly want your naps to be re-energizing then stick to either 20 minutes or 90 minutes at a time.