Feeling light-headed when standing up is commonly caused by a temporary drop in blood pressure known as orthostatic or postural hypotension. When standing, gravity causes blood to pool in the legs, which lowers blood pressure and reduces blood flow to the brain, causing symptoms like dizziness, lightheadedness, and faintness. Normally, the nervous system quickly compensates by increasing heart rate and constricting blood vessels, but if this response is delayed or impaired, the symptoms occur. Common causes include dehydration, prolonged bed rest, certain medications, hormonal imbalances, heart problems, and malfunction of the autonomic nervous system. It can happen occasionally in healthy people or chronically in those with underlying health issues. If frequent or severe, medical evaluation is advisable to rule out serious conditions.