Attached you will find a 1917 illustration making clear as to how the great many soldiers of W.W. I were expected to sleep within the dank, muddy labyrinthine trench system.
It was the preferred plan on both sides that their troops sleep in fields and forests as they briskly march forward to the terror-struck cities of their timid and surrendering foes - but other sleeping arrangements had to made after it was decided that trenches were necessary. Officers in forward trenches would sleep in shifts within muddy little rooms called "dugouts" and the rankers would get something worse - misleadingly they were dubbed, "shelters" and they were simply rectangular caves carved into the walls of the trench. The attached, well illustrated page addresses this topic; itis from a 1917 book titled, TRENCH WARFARE:
"It should be raised at least a foot above the floor level in the trench to prevent water from the trench floor from coming in..."