When people stand up after squatting or sitting for a prolonged period, especially if the blood pressure drops suddenly upon standing, their eyes might appear darker or vision may blur temporarily. This phenomenon is usually due to a condition called orthostatic hypotension, sometimes referred to as postural hypotension.

Here’s what happens:

1. Blood Pressure Shift: When you change position from sitting or squatting to standing, gravity causes blood to pool in the lower extremities, reducing the amount of blood returning to the heart and subsequently being pumped to the brain.
2. Decreased Blood Flow to Eyes: With less blood flow to the head, the blood vessels in the eyes may constrict to try to maintain proper blood pressure in the brain. This constriction can make the eyes look paler or less red, thus appearing darker.
3. Reduced Oxygen Supply: If the blood supply to the eyes decreases significantly, this can momentarily affect vision because the retina requires a constant supply of oxygen and nutrients.
4. Adrenaline Release: The body detects the drop in blood pressure and may release adrenaline, which can narrow blood vessels further, contributing to the appearance of darkness around the eyes.

The body usually compensates quickly through various physiological mechanisms, such as increasing heart rate and contracting blood vessels in the legs, to normalize blood pressure. However, those with certain medical conditions (like diabetes, dehydration, or autonomic nervous system disorders), older adults, or those taking certain medications may experience more pronounced or prolonged symptoms.

If the darkness persists or is accompanied by dizziness, fainting, or other symptoms, it’s advisable to consult a healthcare provider as it could indicate an underlying health issue.