I definitely have suffered from fatigue and I think there are a couple of culprits that caused it for me. First was the pain. Constant, intense pain just drains you. The other is the lack of sleep many of us suffer from. Your body needs rest and we often don't get enough of it because of the pain and the disruptions in our sleep cycles. It's made a huge difference since I started getting 6 or more hours of consecutive sleep most nights. I do not fatigue as quickly and have a lot more energy. It's easy to under estimate how big of an impact this has on our lives.