You Get Light-Headed When You Stand Up Quickly
The explanation: You could be mildly dehydrated. Or you might have orthostatic hypotension (a.k.a. postural hypotension), which occurs when blood rushes to your feet and away from your head as you stand up suddenly. (People with low blood pressure can be especially prone to this phenomenon.)
The fix: Drink plenty of fluids and be sure that when you stand up, you do it slowly, says Donnica Moore, a physician in Far Hills, New Jersey. If you see stars anyway, grab a table or a chair to stabilize yourself or sit back down.
When to see a doctor: If the light-headedness persists or if you actually faint.