Lately, I've noticed that I keep temporarily blacking out when I stand up too quickly. Dizzyness, or "seeing stars" can be caused by many factors. One of these is postural (orthostatic) hypotension, which is a decrease in blood flow to your brain due to a drop in blood pressure from sitting or standing up. If you have low blood pressure, you are more likely to experience postural hypotension.

Low blood pressure, or hypotension, means that you have low blood volume. If there isn't enough fluid volume in your blood vessels, then your circulatory reflexes can't restore normal blood pressure as quickly.

To help understand postural hypotension, pretend your body is a column of fluid. When you are lying down, the column is level, and the fluid pressure is equally dispersed from head to toe. But when you sit up, the fluid column becomes upright, making the fluid pressure rise at the bottom of the column and fall at the top. If this pressure drop isn't corrected, the blood flow to your brain will drop, and you'll get dizzy.

The best ways to deal with dizzyness are to lie down, lower your head, drink water, and/or sit and put your head between your knees.

For more information, see:
http://www.msaweb.co.uk/posturalhypotension.pdf and
http://www.drdaveanddee.com/hypotension.html.

-Alix