There are several things that all soldiers, of every nation, and every era of history have shared in common. One of these is the ability to take a nap wherever and whenever the opportunity arises. When I was in the Army, I knew a guy who could literally fall asleep standing up. I saw him do it several times. He would find something to lean on, and then doze off like he was in his own bed at home.
I guess reenactors like the one pictured above are no different than their 1860s predecessors. The ability to steal a nap between battles just adds to the realism.