Vertigo occurs when you're lying down do to sudden changes in movement of your head and it's known as Benign paroxysmal positional Vertigo.
When you move your head around the fluid that is in your ears also moves around which can trigger the Positional Vertigo when you're laying down.
And when you finally lay down and are still the fluid is still moving around a bit in your ears which is causing the Vertigo as long as you do not have any other underlying conditions.
To prevent Vertigo when laying down try getting into bed a bit slower and slowly lay your head down to prevent the fluid in your ears moving around too much.
Another option to get rid of Vertigo and prevent Vertigo when laying down is to sit down in your bed.
Turn your head 45 degrees in the direction of the affected ear.
Then keeping your head turned, lie down with your shoulders on your pillow and your head slightly reclined over its edge (remain in this position for 30 seconds)
Then go to sleep as normal and the Vertigo should not happen or should go away soon.