How to sleep in a contubernium

[Densus]

To the contrary. In fact, one single stretch of sleep is a modern invention. Segmented sleep, or twosleep, was the normal thing to do for milennia, until the invetion of street lighting. people still slept for about 8 hours, but they woke up inbetween to read, pray, or even go visit the neighbours. Sexual intercourse was also more popular after the first sleep. By the later 19th century this way of sleeping began to disappear.
http://en.wikipedia.org/wiki/Segmented_sleep
So it's even likely that there were men awake while others slept, or even the whole tent could be busy halfway the night! Difficult for us to understand, the practice died out and we can hardly imagine this.

I hadn't heard of this before and it is an interesting concept. But when you are talking about maintaining a guard around a perimeter all night it isn't as simple as a sleep, followed by a period awake, followed by a sleep. The first pair on duty would be awake for a period then have the rest of the night off, the last pair would have most of the night off followed by a period of duty before dawn etc. So you would still be messing with their normal sleeping pattern every night and suffering the effects of that. Doing it over a period of months when my children were babies didn't make it feel any more normal.

If you're really tired this won't wake you up, is my experience.

It certainly isn't my experience. I have been woken by sentries changing over in bashas, tents, portacabins and shipping containers especially when we have been quite densely packed.

A simple duty roster would take care of that. Each contubernium would know who was to stand guard at what our. Each man would wake the next on duty. Officers who controlled them would of course also have their watch duties, so I don't see their sleep being interrupted either. They don't have to wake up to check up on each change of the guard, that's the job of the officer on duty.

That is a workable solution too but it isn't without its problems. If the duty officer is from Century A and he finds there are two sentries missing in the sector manned by Century C how does he find out who is meant to be there? If the off going sentries have to wake the oncoming ones in every tent you would be looking at the number of sentries actually on the wall dropping by 50% immediately before each change over as one of them goes to wake the next pair. To my modern head having a full sub-unit or sub-units on duty at a time is much more simple, easier to control, easier to ensure constant coverage and more logical. I do get that doesn't necessarily make it more logical to a Roman mind.

Sure, but that would leave the question as to what unit could guard the whole camp/fort/site? How many men would have to be on guard at any one time?

Sub-units would do it on a rotational basis. In a permanent base the command element would know exactly how many men it took to ensure adequate security and in marching camps a formula for how many sub-units were needed for a particular length of perimeter would be very easy to work out. 25% of the unit manning the walls at all times seems excessive to me.