Can a toddler sleep too much?

My 21 mo old son has been sleeping about 12 hrs at night and taking 4 hr naps in the afternoon (I usually have to wake him from his nap). I know I shouldn't complain after seeing all the posts about how to get a kid to sleep! But I am a bit concerned. He doesn't appear to be sick or have any type of ailments that would lead me to think something is wrong.