The sensation of your fingers or toes falling asleep, or being pricked by hundreds of pins and needles, is called paraesthesia. The condition generally affects hands and feet, though your limbs might experience it, as well.
Paraesthesia starts with numbness in the fingers and toes. The digits can feel very heavy and be difficult or painful to move. Once you do move them, you often feel a sensation of "pins and needles"--prickling and tingling. After you move for a while, the fingers and toes generally feel normal again.
The most common cause of paraesthesia is a reduced blood supply to a nerve or excessive pressure on a nerve. Alcohol consumption, certain medications and other toxic substances can cause paraesthesia. If you have had nerve injuries, you are more likely to have paraesthesia. Carpal tunnel syndrome, in which a nerve is entrapped, can cause paraesthesia accompanied by pain.
Paraesthesia can indicate that a person has diabetes, hypothryoidism or multiple sclerosis. Diagnosis is carried out by looking for underlying symptoms. Medical history, physical exams and laboratory tests are necessary for a proper diagnosis.
A hand falling asleep occasionally should not be cause for concern. However, if this occurs fairly often, and you have not been diagnosed with a disease that could cause this, consult your doctor. The pins and needles could indicate an undiagnosed problem.