I’ll intellectually/emotionally/physically hard as answers. For me its either 12 hours straight “punching tubes” on a very large scotch marine firetube boiler at the beginning of my career or Easter around a decade ago when I was working with troubled teens and had to engage in 5 separate protective holds in one 16 hour double shift. The former was all physical and the latter was a combination of emotional and physical.
18 hours on a solar power plant in ~120° F heat replacing burnt up reactors and busbars in the inverters, covered in glycol which is like an antifreeze that is real sticky and somehow smells / tastes sweet (but is still gross af) so all the bugs are attracted to you and crawl all over.