I used to be able to work 16, 24 or more hours without stop. Physically or mentally demanding tasks. Without eating, drinking, abusing substances etc.
Now I have a problem to sustain 8-12 hours of physical work that is not that much demanding, because a lot of time I must manage some people online of offline, so I have pauses. Just few hours of helping to carpenters, painting or crimpling cables and I am done and often get sick. Even when I work physically less hours than 8, I need to take an afternoon nap.
I do not know whether I am getting old, or sick, or I stretched something too far before or I am stretching something too far right now, because the multitasking is hard. While multitasking one never gets job 100% and that needs an extra energy, because one needs to constantly contemplate why there are so many errors.
The drop of abilities or of my physical wellbeing was abrupt and it is deep. Today was a hard day. I had to work over 12 hours, because mice started to eat both cables and my car (and I did not sleep during previous night, because I was guarding my cat against pumas [the cat was killing mice]). But at the end of the work twice suddenly started weird sound in my ear and I almost fainted.
Other workers of my age around me work harder than me and they do not seem to have those problems, so I guess it is not just the age. But maybe it is individual.
If you talk to God, you are praying; If God talks to you, you have schizophrenia.