This can be called postural hypotension. Some effects that are felt from postural hypotension is light headedness, dizziness, and faintness. This is a result of a drop in blood pressure when you stand up too quickly from lying down. It is very common and everyone has probably felt this at some point in their lives.
Some people, however, have chronic postural hypotension. This poses dangers in falling and getting hurt. In addition, if this happens frequently, it could be a sign that something else is wrong.
The main cause of postural hypotension is depleted blood volume which means there is less oxygen to the brain. Many times the cause is unknown.
What you are describing is called orthostatic hypotension. Normally, blood pressure is lowest when you are lying down. When you sit up the blood pressure rises slightly and then when you stand the BP rises a little more. When some people rise to quickly the vascular system can not raise the BP as fast as needed, this causes you to feel dizzy or lightheaded because insufficient amounts of blood and oxygen are in the cranial blood vessels. Some may actually pass out--called syncope.
Orthostatic hypotension has a couple of causes. The most common is a medication side effect. Many medications have this as a potential side effect. Another cause is that you are bleeding somewhere internally, usually the the GI tract. You could also have a vascular tone problem.
If you are not taking any medication or not taking one with this side effect, you should seek out the help of your doctor because orthostatic hypotension is never normal.
The basic answer here is that, when you stand up very quickly (especially if you have been lying down), you do not get enough blood flowing too your brain. This lack of blood makes you feel dizzy.
When you are lying down, your heart is not really having to pump your blood upwards to your brain. Your brain is at more or less the same level as your heart. But then if you stand up quickly, your heart and your circulatory system must somehow compensate and start getting your blood uphill to your brain, fighting gravity. This takes a little bit of time. In the meantime, you get dizzy.
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All people may not always feel dizzy when they stand up quickly.However some people may experience this sensation some times depending on their health condition and, how quickly they stand up, and their body condition just before the standing up. This happens because of sudden and temporary reduction of blood supply to the brain.
When we are sitting or lying down, our body is relaxed and the body consumes minimum energy and oxygen. Thus the heart also has to supply less blood to the body and therefore beats at lower rate. Also, when the we are lying down, the heart and brain being at almost same level, the heart has to make less effort to pump blood to the brain. However when we suddenly stand up from a relaxing position the body reacts internally, preparing for action. Thus the oxygen consumption of the entire body, along with that of the brain increases. With this the heart also need to immediately start pumping additional blood. But the heart may take some time to adjust to the requirement of increased blood supply. During this adjustment period, the brain, being located a higher level than heart, may not receive adequate blood and, with that, oxygen supply. This lack of enough oxygen to the brain makes a person feel dizzy.