In the British Army at any rate, a soldier's guard duty lasts 24 hours. In that time he won't be continuously at his sentry post, and in his off spells he is allowed to sleep, so the guardroom has beds, on which soldiers stretch out fully clothed except for boots.
I think the sentries at places like Buckingham Palace are only on for an hour at a time, so they will have several spells on and off over a 24 hour period. In charge of the guard there will be an NCO, normally a Corporal, and from time to time there will be visits from the Orderly Officer, whose job is basically to be available in case there is anything the guardroom staff can't deal with themselves. He should also be present at the initial guard-mounting, visit the cookhouse at mealtimes and inspect the armoury and see that all weapons and ammunition are accounted for at least once during his tour of duty. Normally there will be a Warrant Officer in charge of the armoury who will be there when the armoury is open.
The guardroom may also have cells, so if you want to include a defaulter in your vignette, you can.
Ann