Hazel
                    
                    
                    
                    
                    
                    
                    
                        
                        
                        They don't seem to know what causes it - I wonder about relationship to stress.  It does sap your energy - when I had it I had to take a long leave of absence from my job and was ordered by Dr. to nap every day.  Exerse and walking was out.  Good luck.