Peachey's two first sentences are right, but I think what's meant here is that one is not in bed, and then hops up in bed... to sleep. Thus 'in'. In = contained, under the sheets. On = surface. Sleep on the floor and in the bed, on the bench and in the water.

"In bed" means you are under the bedsheets (probably sleeping). "On bed" means you have not gone under the sheets, and aren't thinking about sleeping. So "hop up in bed" means you were sleeping - under bedsheets - and woke up (or were woken up) abruptly.

Hop up into bed. < Usually said to children before they go to sleep. Hop up onto bed. < It's the same thing. (say either and kids go to bed) It's "UP" because kids are small and our beds are raised off the floor.