What causes the sensation of everything going black when standing up?

Experiencing sudden visual darkening upon standing may be caused by orthostatic hypotension, insufficient cerebral blood supply, or increased blood viscosity.

1. Orthostatic Hypotension

When transitioning from a supine to an upright position, a significant volume of blood pools in the abdominal cavity and extremities, resulting in reduced venous return to the heart. Consequently, cardiac output decreases. If the body fails to compensate adequately for this hemodynamic change, orthostatic hypotension may occur, leading to symptoms such as transient visual darkening (blacking out), nausea, and headache upon standing.

2. Insufficient Cerebral Blood Supply

Poor lifestyle and dietary habits, improper sitting posture, or underlying conditions such as cerebral arteriosclerosis or cervical spondylosis can impair cerebral perfusion. This restriction in cerebral arterial circulation reduces blood flow to the brain, potentially causing transient visual darkening upon standing, dizziness, and fatigue.

3. Increased Blood Viscosity

Elevated blood viscosity slows systemic blood flow, resulting in ischemia and hypoxia in vital organs—including the heart and brain—leading to symptoms such as transient visual darkening upon standing, dizziness, headache, and chest tightness. In severe cases, it may promote thrombus formation, increasing the risk of acute myocardial infarction or ischemic stroke.