Part of the fun of the character is that Snoopy is both a person and not, and Schulz can decide for himself which he is more like. When one is expected and the other provided, there is humor in that moment.
This is a weird place in the development of Snoopy's visual development. He's thicker here than in the years to come. He gets longer and leaner for a while, but afterwards seems to pull back a bit into the "balloon animal" shape of the later years of the strip.
I might have missed one or two, but this is the first time I can recall seeing a single, serif Z representing sleep. Such Zs become an important part of the strip's comic language.
