What a common event. It could be one or both. The numbness, tingling, heavy feeling associated with this could be from pressure and ischemic changes to blood vessels in the affected limb. Because of your body position, you could be effectively and unknowingly clamping off a blood vessel, causing ischemia. It could also be the result of pressure to a nerve or a group of nerves in the area. Nerves seldom work alone. This is an excellent example of why you shouldn't sit or stand motionless for extended periods of time. Try to shift your weight from side to side, doing this will promote circulation and relieve pressure placed on the nerves. Parathesia's are the numbness and tingling felt by the extremity.
On long road trips flex your legs while riding or driving. Make regular stops to get out of the car and walk around for just a few minutes.
Many times when you are sitting in the same position for an extended amount of time you may get a "pins and needles" sensation. Perhaps you have sat with your legs crossed for too long or slept on your arm too long overnight. Many times when this happens it is because the communication between the brain and that part of the body has been cut off. Because of the pressure that is placed on the nerves, they are unable to transmit impulses correctly. The tingling sensation is also a way that our bodies tell us that we need to change our positions.
Nerve impulses carry sensation information from nerve endings in the body to the brain, as well as instructions from the brain to the parts of the body. When you interfere with this transfer by squeezing the nerve pathways, you don't have full feeling in that body part, and your brain has trouble telling the body part what to do.
In addition to this, blood supply has been cut off a bit because of the pressure placed on the body part.
It is blood flow related. The veins rely on body movement as well as valves, to aid in their circulation of carrying blood back to the heart. If an appendage such as your leg "falls asleep" the blood flow is constricted for a time, if you are pressing on it for example. Once you move the leg, the circulation begins again and you feel a sensation of "pins and needles". This is actually the blood flowing freely again in the veins.