Lightheadedness is a reeling sensation, caused by a temporary decrease in blood flow to the brain.
Feeling lightheaded is a sensation that may precede syncope or fainting. It is usually experienced when in an upright position (standing or sitting). If it occurs when you are lying down, then the cause may be more serious. One may have the sense the floor is moving or the room is spinning. Because the sensation is often related to a momentary decreased blood flow to the brain, the sensation can usually be remedied by getting into a horizontal position.
High blood pressure
Abdominal aortic aneurysm
Acute mountain sickness
Transient ischemic attack (TIA)