Those nights, after midnight especially, were always too long for my liking - at some point in time, one gets too old to face the hours after say 0230. Of course, there were nights when I got a pretty good sleep, went out the door in the am with energy. There are some 60ish year old ED docs at Concord Hospital - one works only nights - always good to excellent support, however.
I have always had pretty good stamina for these shifts, but I have noticed that doing back-to-back shifts has taken more out of me as I've hit my 50's. When things get their bleakest, I always try to frame the experience as good training for rogaining.