Prof Horne says: “The test of insufficient sleep is whether you are sleepy in the day or if you remain alert through most of the day.”

In a nutshell, if you sleep for eight hours a night go to work and find yourself lolling and drooling on the keyboard, you aren’t getting enough. If you’re sleeping five hours and running the country, you probably are getting enough.

https://news.bbc.co.uk/1/hi/magazine/6546209.stm