If it keeps up more than 4 or 5 days, I'd get to the ER. May be a blood pressure or blood sugar issue. I have never really heard anyone with this particular complaint on this forum before. I have heard, and personally experienced, light-headedness upon standing when transitioning from SAD to PB during first few months, but I was also getting off BP and cholesterol meds at the time. Extreme dizzyness in middle of night while laying in bed sounds bad.
At the very least, go out and buy a blood pressure machine and home glucose tester. The also sell pulse-ox testers pretty cheap.