So as a veteran of at least a dozen surgical procedures, including 5 joint replacements, I like to remind people that this is a major assault on your body. And I see from your screen name that you are now (or soon hope to be) free from pain. That means your body was at war with pain before, which saps your reserves. Now, it needs to heal from the surgery, repair muscles and nerves, replenish blood cells, and get used to foreign material in your leg. That takes time - after a few of my operations, it was about 6 months.
So be kind to yourself and rest as you need - but if the long naps continue for more than a couple of weeks, you might want to let your doctor know. You may need a "boost" for your red blood cells, or the surgery could have affected something else like your thyroid.
Have you told either your surgeon or primary doc about the need for long naps?
Sue