Interpretation of sleep patterns is a complex thing to determine.
I'm sure in the fine print of any sleep tracker they (Google and Fitbit) would advise that data is only an appropriate record of your sleep patterns. It's the same with step counters: it's not an exact science.
Apparently we sleep in rotations of different levels of sleep consciousness.
There's are some great television documentaries by Dr. Michael Mosley explaining how and how much sleep we really get...Google him.
I don't use any sleep tracker as I don't want to be reminded of how little sleep I had. If I feel good enough in the morning then that's good enough for me.
It's the same with pulse monitoring. Once I panicked as I had a high result then eventually I had a panic-attack which I thought was a heart-attack which resulted in a trip to the hospital.
Sometimes it's just not worth knowing everything.
