My Son Who Is 23 Years Old Sleeps Excessively. He

My son who is 23 years old sleeps excessively. He can sleep 15-20 hours straight, and then be awake for a few hours and go right back to sleep. He comes home from work and takes an hour nap then goes back to work. He had hypothyroidism when he was a baby, but is no longer taking anything for his thyroid. What are some other possibilities that are going on?