A rather fun way to explore collections is to come up with a keyword and see what you get. The Flickr Commons is especially good for this; you really get a whole range!
These photographs, of course, come from me feeling tired one day and thus looking for pictures of people sleeping. Not much more to say!
People napping in Battery Park, NYC, on a hot day c. 1910. Source
Portrait of a couple (June Christy and Bob Cooper), c. 1947. Source
A dragoman asleep on his camel (his 'ship of the desert', in the caption), sometime between 1860 and 1920. Source
Americans sleeping in the London Law Courts, c. 1917-1918 (sadly I'm not sure why). Source
A woman sleeping on a sofa on the street after a fire in Hartlepool, UK, 1922. Source
Man sleeping in a tent. The very best part of this photograph is that this is the same fellow seen wearing a newspaper around his neck in the former post on silliness! (for just that photo, see here) Source
Sailors sleeping on the flight deck of the USS Lexington, 1943. Source
Asleep on a fallen column from the Temple of Palmyra, Syria. 1860-1920. Source