All dogs are different and age differently...although 11 is not ancient for a dog, your dog is now a senior citizen. An old approximation is 1 year for a dog is 7 human years so your dog might be around 77 in human years. Although many 77 year old humans are still very vital and active, I'd bet hard that most have slowed down considerably.
If your dog has seen the vet and gotten a clean bill of health...then yup this just might be old age - and with old age comes lots of napping. Napping is good though, I love a good nap when I can get one
Here's a very general article on senior dogs just in case it can help.
Good luck!
http://www.pets.ca/dogs/articles/senior-dogs/