"Restful" here likely means estimated deep sleep aka Stage 3 delta waves.
We have sleep cycles, roughly every 60 minutes, broken into different levels. Stage 1 (light) Stage 2 (deeper) Stage 3 (Delta) then REM (dreaming).
The restful stuff is the core sleep needed and we get roughly 10-20 mins per cycle. However, most users go in and out of stages throughout the night in different ratios. For instance, we tend to get REM in the later hours of sleep instead of the early ones.
If you're sleeping 7 or 8 hours, then 2 hours of restful sleep is solid.
I'll probably do an article on this for the front page.