Something by that nice James Blunt perhaps 

Sleep cycles last 90 minutes and it's best not to wake in the middle of one. 6 hrs is good. 7.5 hrs is OK. 9 hrs is a no-no. Too many toxins build up in the body with that length of sleep. You will feel groogy for hours afterwards.

Research suggests that if we all got 90 minutes in the afternoon, we would only need 4.5 hrs at night. This is what our ancestors used to naturally do before the industrial revolution came along and messed up our natural cycles.