I agree, check everything about the bird. Also, are his feathers puffed up? This could mean that he has a respritory infection, my zebras got this at one point in the winter becuase of drafts. If you see that his feathers are puffed up and he appears fat and he is still sleeping like that it probably means that he is sick. You can buy medicine for him at the petshop. My zebras are never sleeping only in the night time. You may also want to check with the vet just in case.