I think cats sleep an average of 16 hours a day and on top of that they spend a lot of time watching life go by and contemplating what to do next. Obviously you're keeping an eye on them giving they are acting out of character. If either goes off their food or shows any sign of ill health, then I would suggest a vet's visit (which I appreciate why you're trying to avoid at the mo). How long have you had them by the way and what are their names?