Occasionally, when I stand up after laying flat on a bed or couch I will take a few steps and then start to have tunnel/ darkened vision, a slight headache, and sometimes lose some balance, once falling down and only keeping from hitting the ground by instinct, and then I forgot how I had left my bed for a few seconds.

While I am not to worried about this, I would like to know what might be causing this, so I can try to avoid a more serious accident. Could dehydration cause this? I have some friends who report similar experiences but others deny having them. Are some people more inclined to dizziness from this? I don't think it is from exertion or anything, as I am resonably get and recently completed a weeklong backpacking trip, although it is not something I usually do.

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    Although I love this question and would really like to know the answer to it, I have to tell you that it is out of scope for this site. Questions relating general health are off topic. Feb 6 '14 at 21:29
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    Agreed. It could be something as benign as what the answer suggests, orthostatic hypotension, or it could be a sign of something much more, and there is no way for someone to tell over the internet. Please see a medical professional.'
    – JohnP
    Feb 6 '14 at 22:25
  • Alright, I wasn't sure if this was on topic here either, but as I could not find any other health related Stack Exchange I thought it was best here. As it has not posed a significant risk yet, I will wait until my next routine exam to ask, but thank you!
    – The Ugly
    Feb 7 '14 at 15:34

When the human body goes from laying prone to standing up there is a rapid, and brief, change in blood pressure. This is the likely cause. Some people are more susceptible than others, and it's called orthostatic hypotension, or sometimes, "vertigo."

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