I did some experimenting with this. According to the UK's NHS website, adults need 7-9 hours a day, and it depends on the individual. I started with 9 hours,

I did some experimenting with this. According to the UK's NHS website, adults need 7-9 hours a day, and it depends on the individual. I started with 9 hours, every single night, going to bed and getting up at the same time. After less than a week, I started waking up 30-60 mins before my alarm. So I cut it to 8.5 hours, and then 8 hours etc. With some refinement, I found somewhere just over 8 works. I really don't think it's any more complicated than that for most people. The big problem is sleeping well and socialising a lot. The best approach I've found to that is to bank an extra bit of sleep when you can, although this is controversial. My Dad always reminds me when I'm working hard: "the secret to stamina is sleep." It's a pain, but it's essential if you want to do good work and stay focused.