Why Seven Hours of Sleep Might Be Better Than Eight - WSJ

Latest findings on number of hours of sleep needed.
And, some practical advice on finding it:

Experts say people should be able to figure out their optimal amount of sleep in a trial of three days to a week, ideally while on vacation. Don't use an alarm clock. Go to sleep when you get tired. Avoid too much caffeine or alcohol. And stay off electronic devices a couple of hours before going to bed. During the trial, track your sleep with a diary or a device that records your actual sleep time. If you feel refreshed and awake during the day, you've probably discovered your optimal sleep time.

And

Five healthy adults were placed in what the researchers called Stone Age-like conditions in Germany for more than two months—without electricity, clocks or running water. Participants fell asleep about two hours earlier and got on average 1.5 hours more sleep than was estimated in their normal lives, the study said.

Their average amount of sleep per night: 7.2 hours.

Why Seven Hours of Sleep Might Be Better Than Eight - WSJ