My 4 month old seems tired nearly all of the time. He generally starts yawning and looking glazed over about 30-40 minutes after he wakes up from a nap. I generally try to get him down about 60 minutes after he wakes up, but is sometimes ends up being 90 minutes if he's not going down easily or is extra hungry. Any ideas as to why his wake time seems so short?

He generally gets about 9-10 hours of nighttime sleep (not counting his 2-3 wake ups) and about 4-5 hours of daytime sleep broken up into 4-5 naps.