Stay hydrated, sit up or stand up slowly, if it starts to black out try tightening your abdominal muscles.
JHCHMD
ok so im not athletic at all and it lasts for about five minutes everytime, but it makes sense ... is there anything i can do to prevent this? should i drink more water or something?
You do need to discuss with you MD and see an ophthalmologist but the most common cause is "orthostatic hypotension" which means when you get out of bed or stand up your heart has to work harder becauses the heart goes from the same level as your head to pumping straight up hill. Often it takes a few seconds for the heart to kick in gear, the brain/eye don't get all the blood they need and vision darkens out for a few seconds. This often happens in very young very healthy atheletic individuals with slow heart rates.
JCH MD