SO tired!

I am 5 weeks 3 days and i am always tired! Usually I can get 6-7 hours of sleep and feel totally refreshed by the next morning! Now I can barely get myself out of bed after nine hours of sleep and I take small naps during the day. I work at a retail store so my hours are all over the place. I can barely stay awake. I don't know how I'm going to handle standing on my feet for up to ten hours and only getting a 30 minute lunch.  I am exhausted and am questioning if this job is even worth it!  Any ideas on how to not be so tired?