I also suffer from life long ptsd. Increase in nightmares / sleep disturbances happens to me when i am avoiding dealing with waking triggers, in my case usually with DR/DP, by allowing myself to slip into an unaware state, i guess my brain thinks it is soothing my mind, but it is only a band aid putting the fear on pause... but the unconcious still recieves these triggers and they require some outlet to be felt and processed. Hence increase in nightmares. Thats how i have come to understand it... hope maybe it helps you too.