Just found this, which I found interesting in possibly explaining my issues when falling asleep, don't know if it might help others...
"(Those with PTSD) might feel the need to maintain a high level of vigilance, which can make sleep difficult."
"Hypnagogic jerks refer to involuntary bodily twitches, movements, or muscular jolts that occur during the transition from wakefulness to a sleeping state. Most people are thought to experience them just moments before entering the first stage of sleep."
"Some speculate that hypnagogic jerks are a mechanism that allows your brain to decide whether it truly wants to sleep, or whether it should remain alert and wake up. Without the jerk, you would drift off into a sleeping state without having the option of remaining awake."
"The hypnagogic jerks may serve as a reminder to double-check your environment for potential predators prior to falling asleep."
There seems to be some suggestions that those with PTSD are more susceptible, or may experience hypnic jerks more often.
It makes me wonder, for me at least, if this is to do with my brain reminding me to check for potential threats prior to sleep, like is theorised. I guess, in the case of PTSD it would make sense that we would need to check for threats more vigilantly and could explain that feeling of terror as drifting off, because we know the consequence if a predator is looming.
This appears to be all theoretical, however.