According to chronic pain specialist Dr. Blair Lamb, the sensation of legs or arms falling asleep is usually the result of nerve entrapment or neuropathy. While the condition is often no cause for concern, it can also indicate a stroke, multiple sclerosis, hyperventilation, anxiety, vitamin deficiencies or anemia. Ultimately, many different conditions can combine to cause paresthesia, or the feeling of “pins and needles” associated with a limb falling asleep.
HowStuffWorks explains that paresthesia often occurs when pressure is applied to one part of the body, such as when a person sits or lies down for a prolonged period of time. When this happens, the nerve impulses travelling through the limb become blocked, preventing the brain from feeling the affected limb. This is why limbs that fall asleep are difficult or impossible to control. Additionally, the arteries supplying blood to these nerves are often blocked as well. This prevents the nerves from receiving nutrients and oxygen, which can cause them to perform in strange ways. The combination of blocked neural pathways and blood flow causes the nerves to send unusual and contradictory messages to the brain. This leads to the feeling of “pins and needles.”
According to HowStuffWorks, if such obstructions persist for several hours or more, permanent nerve damage may occur. To avoid this, people should move about or shift positions when they feel their legs falling asleep.