when i'm not feeling emotionally deficient, i usually can go on 6-8 hours and do okay. and that is about how long i just sleep before waking up.
if i'm feeling emotionally strung out and depressed (like i currently am) i'm usually good for little two hour power-naps when i can find time to try to shut my brain down. (it's 3am right now, and i'm working on 2 hours sleep in the last 28 hours, and my brain is not showing signs of stopping. so i'm gonna go read for a few more.) i hate when i get like this, because it just makes it all the more worse, as i start to physically break down too. but good thing is, i'll eventually shut down and pass out somewhere for about 18 hours. then i'm sure i'll be good again... 
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friends are like balloons...once you let them go, you can't get them back.
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To the world you might be one person, but to one person you might be the world.