yea we had this guy called Andrew Fuller come in and give us like stress management skill and all that jazz.
he did say that the maximum you should work a week is 10 hours, nothing more. i only work 2, 5 hour shifts a week, and im not struggling at all, like i used to do like...4 a week...2 being 2 hour shifts, but thats just a waste of time.
i recon if you really wanna work, take the 2 shifts you work the most, and keep at least one day of the weekend free, for a social life and homework and stuff.
also try and work 5-6 hours on a sunday...that way you work the longest AND you get extra pay! (YIEW!)
it works for me, and hopefully it effective for some other people too