First, you should give your vet or an "on call" vet service a call . . . it
seems unusual that would occur this long after the shot.  I would expect any
lethargy before a full day had passed.

Have you tried to wake him, interest him in some food?  Perhaps he is just
tired from his whole new experience.  Did he do a lot of exploring when you
got him home?

When you say "hours" what timeline are we speaking of?  I have a 3 year old
female who does not drink as frequently as my 14 y.o male.  There is nothing
wrong with her.  If she has had vigorous play, she will sleep for 4 or 5
hours at a stretch.

I would try to get your kitty moving, see if you can spark any interest.  If
there appears to be a problem, I would call a vet and see if you can get
some advice.  It may be possible that the vaccine caused a
reaction.............but then again, there was quite a time span between the
injection and your cat's slow-down.  He might just be tired today :)

Let us know!

Melody