The myth of the eight-hour sleep (bbc.com)

I recommend Why We Sleep which says this 2-sleep pattern is not normative for humans. Instead, siesta pattern is.

I wear a Motiv fitness monitor ring and it tracks sleep very accurately. I've noticed over the past year wearing it that the nights that leave me feeling best in the morning usually have a period of 45 minutes to an hour that log as awake time -- usually about half way through my sleep session. Oddly, I rarely recall being awake during this time.