Discipline Equals Freedom Field Manual by Jocko Willink ( PDFDrive.com )

During those breaks, if I got the chance, I would lie
down on the floor, put my feet up on my bed, set my
alarm for six to eight minutes, and sleep. Of course,
I was tired, so I would fall asleep very quickly, and
when the alarm would go off, I would wake up and feel
completely refreshed. It was awesome.
When I got to the SEAL Teams, I used the same
technique and found it especially helpful during long
patrols in the field. When we formed a perimeter for
a break and I wasn’t on security, I would immediately
elevate my feet and take a power nap. The effects
were even more obvious, since on a patrol we were on
our feet for many hours carrying a heavy load.
I still use this effective technique when I am overly
tired.
Warning: Be careful about letting your six- to eightminute
nap turn into a two-hour slumber. If you do
this, you will have trouble falling asleep at night,
which leads to trouble waking up in the morning. That
means there is a higher chance you will fall off the
early morning schedule.