I can cope fairly well on minimal sleep, having 3 no sleeping kids helped with that. But I tend to find if I can get 9-10 hours that's when I'm best refreshed.
I know advice is 7-8 hours for adults (and that lie ins are not recommended, that's it better to keep the exact same bedtimes & waking times), but I also know 7 hours just feels like I've not slept enough.
My children all live happily and energetically on around 5 hours a night. They're 10, 8 & 7.
So, who are these adults that are full of beans on 7-8 hours?