Is OTHER wrong in the post ? POST : Hey all first post here.

 Background info:
I work in a hospitals lab with very little patient interaction and no direct care. We are currently super short staffed (down 1/3 for various reasons ). There are several point age positions ranging from contingent to full time and anything in between.  On average everyone  picks up 4 to 8 extra hours per week to help with the staffing shortages. I myself pick up minimum of 8 scheduled hours per week usually 12( i work nights so i come in 4 hrs early two to three times a week) 

The problem. 
I want to leave when im supposed to leave. If im scheduled to leave at 7 i want to be clocked out by 7:05 (barring any projects, tasks , emergencies) tue problem is when morning shift comes in and takes their sweet time starting to work( morning chit chat, coffee, slow and waking up ). So the morning rounds begin at 4 and my shift ends at 7. Often i will begin the work at 3 to guarantee it is done or reasonably complete by 7 (the bulk 90+ % done, work always trickles into the lab ) before i leave. We have these older coworkers who take their morning breaks religiously ( not lunch breaks ) and more often im stuck staying late to help with the trickle of work that comes in after im supposed to leave so they can get their breakfast in. 

Additional info: 
-on nights we only get our 30 min lunch
-numbers alone i do 50-60% of all the morning rounds work, even though there are 2 other people to split the work.
- its usually 30 min to an 1hr after im scheduled to be done that  i get to leave because they go slow and won't pull their weight.

I want to leave when its my time. I don't want to stay to help with routine work or to cover for breaks that i don't get. I don't like the morning shift coworkers as colligues or perosnaly so i hold no desire o help them out of good will and I'm already working  12+ hrs of overtime. 

TLDR; coworkers are slow, and cause me to stay 30 to 60 min late 3 out of 5 days. I want to leave when im scheduled to not when its convenient for them. 