Getting 6 hours of sleep is okay for some people to function through the day and if you're feeling rested with only 6 hours of sleep then it's fine for you to get only 6 hours of sleep.
I try to sleep at least a minimum of 8 hours per night so that I feel much better the next day and on weekends I try to sleep as long as 10 hours.
The more sleep I get the better I feel.
The less sleep I get the worse I feel and when I do get only 6 hours of sleep I feel okay but not as good as I would feel if I had gotten 8 hours of sleep.
Everyone has different sleep needs but you should get at least 6 hours of sleep per night but getting at least 8 hours of sleep per night is better for the best health.