Twice now i have been in bed either waking up or falling asleep and all of a sudden it feels like the whole world is moving up and down. why?

Positional Vertigpo: A possible diagnosis is bppv or benign paroxysmal positional vertigo. This is a common entity thought to be due to loose calcium carbonate crystals in the inner from one of the balance organs called the utricle. When specific movements of the head occur, the loose microscopic stones (otoconia) move and cause a true vertiginous (spinning) sensation. This is often a self limited problem.

Confusional arousal: Could be just hallucinations in sleep transitional phase or parasomnia--need more history.

Vertigo?: Agree with the need for more history- if there is prominent nausea and the spells are triggered by head movement, benign paroxysmal positional vertigo (bppv) is another (treatable!) possibility.

Could be..,,: A hypnogogic hallucination. Simple put you are hanging on to the last remnants of wakefulness and sleepiness at same time. Brain tries to reconcile both inputs. Sometimes it succeeds and sometimes it doesn't and your world bounces. See your doc if there are other symptoms.