40 years of rotating 12 hour shift work takes a toll on the human body that 9 to 5 people who sleep and eat well can't even begin to understand.
In the early 90's I worked 12 hour shifts but always 6pm to 6am. Two days on, 2 off; 3 on, 3 off. No exceptions for week-ends or most holidays. But none of that rotating between day and night shifts.
How you have survived for 40 years is completely beyond me. We had well over 100% turn over rate annually. I was certainly a part of that turn over. Only lasted a couple of months. Couldn't sleep. Was utterly miserable.
A tip of my hat to you, sir.