[dpeckett]

Melatonin has a short half life (~1h), that's why melatonin receptor agonists [1] are a thing. So just mechanistically it's unlikely to help with sleep maintenance.

Do you wake up after 5.5h at a consistent time of the day and the first half of the night is peaceful? If you fall back asleep do you then wake again shortly after?

I mean waking in the night can be many things (apnea, etc), but you could very well have a rather advanced sleep phase.

1. https://en.wikipedia.org/wiki/Melatonin_receptor_agonist