I get a similar dip about an hour before my alarm time.
The odd thing is one of the three days, can see my HR gradually decreasing through the sleep as normal from going to bed just after midnight, getting to the 50’s… I woke up before alarm, laid there awake a bit plus had a toilet visit, so HR is up into 70’s) went back to bed just to wait for alarm (!) and had an almost immediate and sharp dip down into the 40’s before getting up and I was back up to 70’s.
It doesn’t seem like it can be real - especially so soon after being awake. I’m not convinced I even went back to sleep.
Screenshot_20210422-172604_Suunto.jpg
(the blank bit is Watch on charger).
That Oura link is fascinating - mine is a downward slope - and its correct that I ate quite late and was drinking alcohol until very close to midnight (and also correct that I felt a bit off the next morning!).