I used to farm so quite often would be on the go for 18 hours a day esp during lambing, calving or foaling. Harvest time you kept going until the job was done.
Have you been to your GP to check up on bloods to make sure all is in order (thyroid etc.)
Don't work 12 hour shifts now but have done in the past @Nicoledeex , unfortunately shift work does leave you tired and you never catch-up on your sleep, but if you think there's a medical cause for the tiredness then it's best to get a check-up at your Dr's.
I used to do 12 hour shifts, I found the older I got the harder they became. I found it difficult to wind down after a day shift. I found swimming and walking helped, watching tv and going on computer made me stressed.
I found I hit a "wall" after about 8 hours, then about an hour before I finished up became euphorically charged.. It felt like I could "go round again"!?
My wife found me quite hyper when I got home...