Credit: iStock

This week, the I Spy Physiology blog answers a reader question: Why is it hard to sleep upright on an airplane?

Having trouble sleeping when traveling on an airplane or by train or car may not be just because of the change in air pressure or the motion of the vehicle. Your circulation—or lack of it when you’re sitting up—may be the problem.

When you sit for a long time without moving, less blood flows to and from your legs. Normally, muscle contractions in your legs play a large part in returning blood to your heart. But when you sit for long periods, there is no pumping action coming from your legs and the blood has a hard time flowing back to your heart. This causes the blood to pool in your ankles and feet, which can make them swell. Your legs may also become numb and tingly, making it hard for you to get comfortable and fall asleep on an airplane.

Aside from being uncomfortable, you’re in danger of developing blood clots in your legs (a condition called deep vein thrombosis or DVT) when you sit for a long time. Restricting the blood flow to your legs for several hours—such as sitting in a small airplane seat during a flight—can sometimes lead to DVT. If a blood clot loosens from the vein, it can travel to and block blood flow from your lungs and heart. This is a very dangerous situation and can even be fatal if the clots are not treated.

Blood flow to your legs is not only important to supply nutrients and remove waste from the muscles in your legs, but it also helps keep the endothelial cells that line your blood vessels functioning normally. When blood pushes past the walls of your blood vessels, it creates what is known as shear stress. Shear stress helps your blood vessels relax and contract normally and keeps your endothelial cells healthy. Sitting for as little as three hours reduces both blood flow and shear stress, making it harder for your blood vessels to relax. Lower levels of shear stress are associated with many chronic diseases such as atherosclerosis, diabetes, stroke and hypertension, which may explain partly why prolonged sitting puts you at a higher risk for these conditions. Fortunately, studies show that standing up and moving around for a couple of minutes can reverse this type of blood vessel injury.

So, the next time you‘re on a plane or even just watching a long movie, get up for a drink of water or a bathroom break, if only to move your legs.

Dao H. Ho, PhD, is a biomedical research physiologist at Tripler Army Medical Center. The views expressed in this blog post are those of the author and do not reflect the official policy or position of the U.S. Department of the Army, U.S. Department of Defense or the U.S. government.