There is a list of about ten things - roughly, from memory - and you have to have k out of n things up with you.
The most important one is delayed fatigue after some kind of muscle strain or physical exercise or exterior or mental exertion. The classic pattern is for malaise and fatigue, intense tiredness and almost flu-like gritty feelings to kick in so many hours after serious physical or mental exertion. I first noticed something odd when I did a long drive down go England and then felt horrible, completely washed out about 20-30 hours later. I thought it odd that I felt wrecked later rather than immediately after doing such a drive.