Inaccurate HR Reading While Sleeping?

I've had my Series 2 watch for about 8 months now. I've worn it off and on to bed, but most weeks will at least wear it for 3-4 nights to get a better idea of my resting heart rate (usually somewhere between 58-60). A couple nights ago I wore it to bed and it recorded the following data at about 4:00am:

As you can imagine, I'm a little freaked out. I've never seen anything like this in the 8 months I've been wearing it, let alone while I was sleeping, and I didn't wake up suddenly feeling all out of sorts or have any chest pain, etc. I have to think that this is a fluke. If it weren't, I think I would have seen something similar by now if I've been wearing it to bed so consistently.

The day before this occurred, my watch read 144 bpm while I was sitting in a car and I held my hand to my neck to take my pulse and verify it couldn't be accurate. Inaccurate readings tend to happen for me with the series 2 cyclically and only stop after restarting the watch if I've worn it for a couple weeks without doing so.

Full disclosure: I also went to the doctor just in case and they didn't really have much of an explanation except to discount the errant readings. Has this happened to anyone else with the Series 2?