[boothling]

She slept in this bed.
She slept on this bed.

Both are correct, but native English speakers would probably say "in".

You use "in" when somebody is underneath the covers (blanket, duvet, sheet). But you would say "she sat on the bed" if she was on top of the covers.

Irushka:

You use "in bed" (no the or a) if someone is sleeping, or ill, or lying down. For instance:

- Where's Susan? - She's in bed.
- He was still in bed at one o'clock.
- I was sick, so I stayed in bed.
- Shall I bring you coffee in bed?
- She has been in bed for two days.

You could say "I stayed in my bed" or "she is in her bed" and it would be correct, but "in bed" is more usual.