What Causes Alternating Between Sleeplessness And Prolonged Sleep?

Having problems with sleeping seem to wake up in 4 hours of sleep cannot go back to sleep for a couple of hours then go back to sleep sleep for a couple more and continues what do I and some times I will sleep for 12 hours straight need help and have seen my doctors but they don't seem to want to help I am 52 My body has been through a lot when I was 26 I had my spleen removed because of a blood problem I have survived sepsis that I had when 47 all I want is for some one to tell me what I can do ( I have had thyroids check and they were a little low)