Why Can Sitting For A Long Time Cause Blood Clots, But Sleeping For Hours Doesn’t?

Sitting for long periods of time can cause blood clots due to impeded flow of blood. The lack of movement can cause the blood to clot according to the effects of gravity. This doesn’t happen during sleep since we do move our bodies during our night’s sleep. Sleeping can also cause blood clots and bed sores if done for a very long time.

You may have observed that when you sit in the same spot, in pretty much the same position, for even an hour, your legs start to feel…weird. It’s a strange sensation indeed; some feel ticklish, some feel pain or discomfort, some feel an uncontrollable urge to move their feet, and some partially lose sensation in their legs. Some or all of these symptoms begin showing when you sit in the same position for even as little as an hour or two.

The face you make when you have been sitting in your seat for 4 hours without moving a muscle meme

On the other hand, when we sleep, we typically lay down and remain still (for the most part) for 6-7 hours on the bed. Yet, that doesn’t seem to hurt us or result in the kind of symptoms mentioned above.

Sitting for a few hours can cause discomfort or even result in the formation of blood clots (which could potentially be deadly) in the body, but sleeping doesn’t seem to have the same effect on the body. Why is that?

How The Blood Flows

As you know, the heart is the most important part of the human circulatory system. Blood begins its journey from the heart. After exiting the heart, blood enters the aorta and arterial system. Arteries are much thicker than veins, as they consist of thick layers of muscle and elastic fibers.

Arteries being thick is actually very important, as they have to stand the pressure of blood flowing through them. Also, their elasticity comes in handy to push the blood and make it flow smoothly.

Veins & Arteries system in Human body

Arterial vs venous system in the body. (Photo Credit : Pixabay)

After going through capillaries (the most basic and thinnest kind of blood vessels), the blood enters the venous system and heads back to the lungs and the heart.

The venous system contains very floppy vessels with valves that help to prevent the backflow of blood. It has a significantly thinner tunica media (the middle layer of the vein), and as such, it relies on the contraction of the skeletal muscles (muscles that you voluntarily use while walking) for proper blood flow when blood must flow against the pull of gravity (i.e., from the legs up to the heart).

Contracted skeletal muscles relaxed skeletal muscles veins venous valve blood

Notice how the contraction/relaxation of skeletal muscles impacts blood flow in the veins.

Since blood in the veins has to work against gravity to move upwards (towards the heart and lungs), when one remains in a sitting position for too long, the blood may not move fast enough. This could trigger the clotting mechanisms in the body, which could potentially form a blood clot (or in biological terms, a thrombus).

Circulation During Sleep

When you are in the ‘lying down’ position, or the kind of position one has while sleeping, your legs are more or less on the same horizontal plane as your heart. Due to this, the blood doesn’t have to work as hard to fight the gravitational pull to reach the lungs and heart. Hence, the ‘return flow’ of the blood becomes relatively easier.

Girl sleeping position

While sleeping, your legs are more or less in the same horizontal plane as your heart. (Photo Credit : se media / Shutterstock (royalty free)

You do move ever so slightly while sleeping, but that doesn’t cause any blood circulatory problems because your body is mostly straight (flat) and the blood doesn’t have to flow upwards.

However, that doesn’t mean that one is absolutely safe in the ‘lying down’ position indefinitely. Even while lying down, gravity is still the most influential variable to the blood flow. It’s just that while lying down, the gravity variable is partially alleviated, making it less of a problem for 8-15 hours.

That being said, if you stay in the same lying down position without any movement for more than 20 hours or so, you may begin to face problems. That’s why doctors take this issue into consideration while treating bedridden patients so that clots don’t develop in their blood.

Circulation During Sitting

The dynamics change while you’re sitting. While being in the ‘sitting’ position for an extended period (this usually happens when one is traveling in trains, buses or airplanes), skeletal muscle activity is low.

Old man sitting in airplane

Sitting for too long without much movement can cause discomfort and pain. (Photo Credit : Pixabay)

In this position, the legs also aren’t moved very much and they are often angled. As such, the blood flow fights against gravity. Due to certain biological processes that occur, the velocity of blood flow in the upwards direction (against gravity) is reduced. This can potentially lead to the formation of blood clots in the body.

Related Articles
Related Articles

The thing about such a blood clot is that it can float through the veins and reach the lungs, where it could severely damage a part of the lung (a condition called Pulmonary Embolism) before it’s treated. That’s why it’s always recommended to get up frequently and move around a bit while traveling on particularly long flights or bus/train journeys.

Help us make this article better
About the Author

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

Science ABC YouTube Videos

  1. Digestive System: Ingestion to Egestion Explained in Simple WordsDigestive System: Ingestion to Egestion Explained in Simple Words
  2. What is Radioactivity and Is It Always Harmful: Explained in Really Simple WordsWhat is Radioactivity and Is It Always Harmful: Explained in Really Simple Words
  3. What is DNA and How Does it Work?What is DNA and How Does it Work?
  4. USB Types: Various Types of USB Cables (A, B and C) and Their DifferencesUSB Types: Various Types of USB Cables (A, B and C) and Their Differences
  5. Grandfather Paradox: Explained in Simple WordsGrandfather Paradox: Explained in Simple Words
  6. Highest IQ Ever Recorded: Top 16 Smartest People of All Time.Highest IQ Ever Recorded: Top 16 Smartest People of All Time.
  7. What are Mutations and what are the different types of Mutations?What are Mutations and what are the different types of Mutations?
  8. Gravitational Lensing: What It Is And How It Is Helping Us Discover New GalaxiesGravitational Lensing: What It Is And How It Is Helping Us Discover New Galaxies