Melatonin helps you fall asleep but doesn't help you maintain sleep.

Sleep disturbance is very common with PCS, I suffer from similar symptoms as thorx89, I often sleep only 4 or 5 hours at a time and wake up feeling alert unable to get back to sleep at some god forsaken hour. I think this month I've slept more than last month which is progress, but it's approaching 9 months since my injury! I tend to sleep best when I'm in a good mood, which seems to suggest low serotonin which fits my sleep pattern which is similar to someone suffering from depression and anxiety even though I don't feel like that.

I don't know whether PCS sleep disturbance is physical, poor blood circulation in the brain for example, or psychological like post traumatic stress syndrome or some form of residual anxiety. But something triggers waking up before time. Basically if you can't remember dreaming then you probably had a decent nights sleep.