How long you spend in bed versus actually sleeping. 90% sleep efficiency means that you were awake 10% of the time you were in bed.
Mid sleep hour is the middle hour of your sleep. I.e. sleeping from 10PM to 6AM, midsleep hour would be 2AM. This is a nice simple indicator of inclination to one of the two chronotypes.
Blood oxygen saturation. Tells how much oxygen is in your blood, where 100% is the maximum oxygen that can be there. Normal value is around 98-99%.
Respiratory Disturbance Index: the average count of respiratory distrubance episodes per hour. E.g. if you stop breathing once every hour of your sleep, RDI would be 1.
Some accelerometric sensors are able to gather data in themselves and then send them out in batches, which largely reduces battery consumption by Android reading the data. So effectively sensor batching reduces battery drain of the sensor reading to 1/5.