I use this system:
It takes half an hour before they sleep so deep that they get PP.
For every 2 hours of sleep they get 1/3 of their PP back.
However if they are awaken from sleep (to fight or false alarm or whatever) they need half an hour to fall asleep again and then they still need half an hour more (for one hour total) before the PP starts comming again.
This is the best compromise I have found between realism and playability.. and it gives me the option of ruining their sleep once in a while, if i need some tension in the game
It's not perfect and if somebody have a better system, please post it