on/in (the/a) bed

  1. He is sleeping in bed.
  2. He is sleeping in a bed.
  3. He is sleeping in the bed.
  4. He is sleeping on bed.
  5. He is sleeping on a bed.
  6. He is sleeping on the bed.
    Which of the above sentences is NOT acceptable?

#4 is ungrammatical

#4-6 are unusual. You could use them if ‘he’ is a dog.