A: When you stand up, the effects of gravity on your body can cause your blood to settle or pool in the veins in your lower body. Normally your body has a number of immediate responses that helps protect against this drop in your blood pressure. If any of these responses are off or slow to happen, the result is that your blood pressure in your brain drops which causes you to feel dizzy.
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Being careful when changing position suddenly, such as bending over and standing up or standing from seated position can help. If the episode is severe, like you feel like you will pass out or if the dizziness doesn’t pass quickly, sit or lie down right away.
Be particularly careful when getting up from bed. Sit on the edge of the bed for a couple seconds before standing and take a couple seconds before you start walking to give your body a chance to respond. Similarly, use caution in the morning or after prolonged periods or lying down, when getting up in the middle of the night to use the restroom, after large meals and after hot showers.
Many things can affect how you regulate your blood pressure when you stand. Things such as age, medications, medical problems, dehydration, and even deconditioning can be associated with postural dizziness. If this is happening often, see your doctor.
— Cardiologist Jeffery Courson, DO