Schengen Zone is the same zone where you will board your flight to AMS. When you arrive in AMS, you are still in the Schengen zone but after you pass through passport control to connect to your flight to MSP, you will be inside non-Schengen zone. Another way of looking at it is, Schengen = domestic + European flights (except UK/Ireland); non-Schengen = international (including UK/Ireland).

I don't know if security will allow you to enter the boarding area before midnight but, in any case, there are limited number of sleep pods and most of them will probably be taken when you get there.