I've been getting dizzy spells and feel nauseous only when i lay down. What could this be?

Last night I experienced an unpleasant dizzy spell in my sleep (it woke me). It was the kind of feeling you get when you are vey drunk and it feels as though the world is spinning around you. Then just now I was laying down and it occured again, this time making me feel nauseous. Both times the dizzy spell didn't last for long - 20-30 seconds. For the past month I've completely cut sugar from my diet, except sugars from fruit and honey. But I don't eat anything with refined sugar - no soft drinks, cakes, sweets, chocolate. I'm 40 years old, male, fit and have a healthy diet. I drink alcohol very rarely and I don't smoke.