Its not healthy for you just sleep 6 hours a day, you better make it 8 hours so your body still fresh even you 40s
sleep various for everyone, some people need alot, some a little, some 8 hours, etc.. if you are tired still when you get up after six hours of sleep then its not enough. so goto bed early or stay in bed later. six hours is more than enough for me, i'm a health nut, so i can function normally on that amount.
Most of the successful people like Donald trump Bill Gates sleeps only for 4 -5 hours a day. The rest of their time after working certain hours in their own company they will be financially educating themselves. I think if you are actually sacrificing your sleep in order to achieve something it's alright. Some things have to be sacrified in order to succeed. It's just my honest point of view.
I think it's enough for about 5 days a week and then the other two days i'd try and get 9+ to catch up for it. You don't want to strain your body as it can cause health issues - and we don't want that .
You should know it when you wake up. If you have trouble getting out of your bed, it's probably because you need to sleep a little more.
It's not the same for everyone but 6.5 to 7.5 is optimal in most cases. It all depends on how fast you get into REM sleep.
Recommended amount of sleep is 6-8 hours per night. I read an article in which there was a study done, and the results showed that people who got 6-7 hours of sleep lived longer lives than those who got 8 hours or more.