Great info from Dr. Matt Walker.
If you want to hit the "sweet spot" of around 7 hours of sleep per night, you probably have to be in bed for substantially longer (depending on your efficiency).
And if you use the median of that range, and assume high sleep efficiency numbers (i.e., strong, healthy sleepers), the amount of time needed to be in bed to obtain that median essential amount of sleep looks like this, as best I can tell.