Fast Asleep or Sleeping Fast?

Every morning Spencer is the first person in our house awake. And every morning we have to say to him....Shhhhh Kaylee's still fast asleep. Wel this morning he came into our bedroom and as he and Lee were getting ready to go downstairs he said to Lee.... Shhhh Kaylee is sleeping really fast. Lee said you mean fast asleep? Spencer said no sleeping really really fast. He's a cutie.