sleepytime

If you regularly wake up in the middle of the night and can't get back to sleep quickly, the break between your 2 sleeps could all be very natural. 

It's believed this is the way we slept before we got artificial lights, drank lots of caffeine and stayed up way beyond our bedtime.  And research indicates it's how our bodies naturally want to sleep when we have proper darkness at night. 

Here's the article.

My sleep cycles are around 80 minutes and I need just over 8 hours a night.  So when I next wake in the middle of the night and don't doze off again quickly, I can relax knowing the second 4 hour sleep is on its way.

And simply knowing that will help me get back to sleep anyway.....