If I want to do yoga before going to work, I wake up at around 7AM. But during weekends, I sleep until about 10 in the morning.
6 am is the best time to wake up. for that to happen, one shd sleep at 10 pm. i do that everyday (except when i am out partying) and feel wonderful for the whole day.
The best time to wake up is between 7 am to 8 am. But it is not fixed up. But it depends, for example, if you sleep at 2:00 am, then you can wake up at 10:00 am, because you have to sleep at least 8 hours a day.
I'm pretty sure everyone is different, though many many people will swear by a 6:00am (or pre-dawn) wake-up. There are sleep studies that can help determine your personal optimum waking time.