Yesterday I talked to a friend. He is perfectly healthy, has no allergies, does lots of sports and is in perfect shape. When we talked about his work, he told me something that doesn't go out of my head. Despite the fact that he is perfectly healthy, he said, that there are days, where he has to get up very early in the morning, works (physical) and gets home late at night. He has normal meals but he is so tired then, that he goes to bed early. He doesn't even want to watch TV. His fatigue seems to be very much like PWCs fatigue. In this state, additional tasks just wear him out. In contrast to PWCs however, after one night of sleep he is back at 100% energy. Another thing he said was that when he gets not enough sleep for several days he will suffer from bad concentration and fatigue during the day. Not exactly like brain fog but going into this direction. He is less able to accomplish his tasks as good as he normally does. So in the end, it would be very interesting to know what fatigue actually is and why the bodies of healthy people can suffer from the same fatigue as PWCs (if they work hard enough) do but are able to overcome it within one night. What's the difference? For me it seems that we don't even know what fatigue actually is. ATP, mitochondria, hormones, neurotransmitters?