why do I experience such debilitating fatigue with my arthritis?

before an attack of my arthritis I become so tired I can not function normally. I usually must nap. Although sometimes this helps often it does not and the tired feeling remains. I feel fine in the morning, although I am stiff, and by afternoon I am too tired to go on. This does not happen every day, only when I am having a recurrance. I am 69 years old and first experienced arthritis 10 years ago about two years after a rash on my body was diagnosed as psorisis. It hit me hard. I had been taking paxil for depression for two months prior. I found I had to nap for several hours/day. After several weeks of napping I experienced pain so great I struggled to get to my car. Getting into my house was very painful. I couldnt climb the steps. This was freightening because the pain came so suddenly. The fatigue stayed with me. I went to a rhumetologist who put me on prednisome for ten days and asulfadine which I took for two years. I am much better now, however I still tire easily. I had two remissions and have not had any serious down-time for 3 years. Last time I was giving myself injections. Then I regained strength again and stopped these drugs. Its as if this hits me out of the blue and dissapears the same way it hit. Exercize helps considerably. I am currently taking no drugs unless I have two advil for pain. A warm climate, rest, sunshine help a great deal. But I still do not understand the reason for the fatigue. thank you for any help you can send. I am also wondering about remissions. Why does this go away all of a sudden and then return. I am hoping to stay relatively healthy (if I do not exercize and sleep well I am achy).