Deadly heart attacks are more common on a Monday

Used to wonder why more hospital patients had critical episodes at night. My brother-in-law who's worked hospitals all his life, said simply "the night shift is on"

not entirely sure what this comment is saying. Night shift workers not as effective? Can someone spell it out for me?

I'm going to presume the OP was hinting at people working night shifts are more sleep deprived, and thus more accident prone, causing the increase in emergency room visits.