This is the reason I got one. To track sleep patterns. What wrist do you wear it on? On the Fitbit flex you can change dominant wrist settings so it will be more accurate.
Only thing I really don't like about them is the cheesy buckle. Just snaps together and has twice been knocked loose on the job. Fixed this by wrapping black tape around the band and a heavy rubber band. Rubber band also goes around my wrist. All can still be removed, but won't just fall off and get broke or lost.
My stepdad sells chemicals to Foxconn. He was telling me about their facility. They call it something like Foxconn city. Came home after one trip in disbelief. They put up nets between a bunch of the buildings so people couldn't jump. Said it was the creepiest thing he's ever seen.