Usually this happens when blood supply is interrupted for a short time, and is reversible (a condition called paresthesis). It happens when your position causes an artery to kink, like a garden hose, which blocks blood flow to the affected appendage. Moving the appendage to another position usually restores blood flow, and they "wake up". Sometimes this condition can be caused by nerve damage. Trauma, uncontrolled Diabetes, among other issues can damage the nerves that allow you to sense your arm (or hand). This is usually more difficult to remedy. It can take time for nerves to heal after trauma, and often times diabetic neuropathy is irreversible. This is one of the many reasons it is so important to control your blood sugar level if you have diabetes.
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If you do not move any part of your body for a period of time it will "fall asleep".
This basically means if you are leaning on your hand or havent moved it for a while the blood flow becomes restricted and the nerves start to "go to sleep" and stop firing electrical impulses. This is why you get pins and needles. It is because when you move your hand the nerves are re awakened and begin to fire randomly :)
when your hand is hanging over something for too long all the blood goes to the tips of your fingers , which causes your hand to fall asleep
This sounds like it may be a pinched nerve. But something like this can be something more serious. Any time a person looses sensation, they should seek medical advice.