Sleep goes in about 90 minute cycles. From light to deep sleep to light again. Wouldn't it make sense that the optimum sleep at night would be a multiple of that?
For optimum sleep, you should wake up after 1.5 hours, 3 hours, 4.5 hours, 6 hours, 7.5 hours, or 9 hours. Right?
My (unprofessional) guess is that the cycle probably continues during waking hours. So the best time for a nap might be at 4.5 hours after waking, 6 hours after waking, or 7.5 hours after waking..
Finally, when taking a power nap, one should either wake up prior to deep sleep (some amount of time less than 25 minutes) or after a cycle (90 minutes).
The great thing about all this is you can test it for yourself easily!
