Caporegime
Just before 11pm to hopefully be asleep for that time. Wake at 7am.
8 hours of sleep is mandatory for good health, it's the bedrock of every other aspect of health. Reading this book has put the fear of God into me. Highly recommended.
8 hours is not a hard and fast rule though, there is a range that averages 8 hours, some people are fine on 7, others ideally need 9, etc.
And that doesn't even bring quality in to the mix, 6 hours of quality sleep is worth more to your health than 9 hours of poor sleep.