You're tired from your long shift and are doing sit-ups in bed? Have fun in boot camp. That's called being a lazy ****. No ones gonna care in basic that you got your ass handed to you all day long and then wake you up at 4am for a fire drill, only to get your ass handed to you again.
Most of us here work long jobs AND train. Sone of us in the military too.
My advice- Quit thinking doing sit-ups in bed will benefit your life in any way, shape, or form. Get up and get used to running and endurance