@Piss_Ant Is it falling asleep that's the problem, or being woken by nightmares? If it's the former, some of the older (pre SSRI) anti depressants cause 'drowsiness' so can help with this if taken at night. They're helpful for some of the waking-hours ptsd symptoms too.
(I put drowsiness in scare quotes, as for the first few weeks of taking these, it was like someone hit my off switch about 20 mins after taking them. No gradual sleepiness, just 'Oh shit, gonna fall asleep, gotta lie down NOW!)