I don’t know what the set-up is at the Quantico Brig so I can’t comment directly on their procedures.
I worked in NY prisons. We allowed people in special confinement to sleep whenever they wanted. A lot of them ended up sleeping during the day and staying awake at night. It wasn’t an issue because we had pretty much the same coverage on all shifts in special hosuing areas.
But we would enforce sleeping schedules in general population for pragmatic reasons. We would have reduced security at night based on the assumption that most prisoners would be asleep. If we allowed prisoners to sleep whenever they wanted to, some would sleep at night and some would sleep at day and we’d end up having to provide full coverage around the clock. So we regulated their schedule so everyone slept at the same time and part of this was forbidding daytime sleeping.