JWPs was a reasonable question but perhaps AF447 will put into context the practicality of "switching off" for a rest during the "quiet" cruise phase. Their problem started at 02:10'05 when the autopilot disengaged and less than 5 mins later at approx 02:14'28 the aircraft crashed.
I will only nap if the alternative is I will fall asleep anyway. This method should not be used procedurally.