Engaging in any behavior off the job, given that you've got a decent time frame (10-12hrs) prior to start of shift, is totally acceptable. As long as you are 100% mentally there, you can give your best to care for your patients while on duty. I would never go out drinking all night, catch 3-4 hrs, and then start a 12hr shift all hungover and sleep deprived. I work in the ER and boy can I tell you how important it is to be A&Ox3 with a GCS of 15. I'm not supplementing nursing with a wild alternative lifestyle, I was just curious to hear from others in the field.
When you really think about what is at the core of this thread, it boils down to: Is this person competent? Is this person cognitively alert? Honestly, if I ever had to undergo any type of procedure, I would easy choose a surgeon with the greatest success rate (with the smoking, drinking, cowboy lifestyle) over an average surgeon (with the church-going, tight whitey, everything by the book mindset).
Know your ****, bring your A game, and have fun!