Why does it hurt when blood rushes back into a arm after falls asleep?

The tingling, pins-and-needles sensation you feel when blood rushes back into an arm that has "fallen asleep" is caused by a temporary interruption of blood flow to the affected area. When you sit or lie in a position that puts pressure on a nerve, it can compress the blood vessels and restrict blood flow. This lack of oxygen and nutrients to the nerve cells causes them to stop functioning properly, leading to the numbness and tingling sensation.

When you change position and blood flow is restored, the nerve cells start to function again. This can cause a tingling or prickling sensation as the nerves "wake up" and begin to send signals to the brain. The sensation usually goes away within a few minutes as blood flow returns to normal and the nerve cells fully recover.

It's important to note that while the sensation can be uncomfortable, it is usually harmless and does not cause any lasting damage. If you experience frequent numbness or tingling in your arms or legs that is not related to a specific position, it's a good idea to see a doctor to rule out any underlying medical conditions.