For me it manifests itself in headaches (sometimes) and low back pain but especially fatigue. Like, overwhelming fatigue as opposed to just tiredness. I feel like I can't do anything. At its worst I can sleep for over 12 hours. I've awakened after 10 hours only to take a shower and make some food and then find that so exhausting that I've gone back to bed to sleep for another 8 hours or so. That's at its worst.