Passing out?

I recently attempted a 7 day fast, however, was only able to fast 4 of those days, as on the 5 day I passed out.

I've tried to do this multiple times and I can't seem to get passed 3 or 4 days. I'm drinking lots of water and I get up really slowly and am cautious.

I'm average weight: 5'8 and 140 pounds. What am I doing wrong?