7-9 hours of sleep per night may well be healthier than sleeping less than seven hours per night, there may be people who will be born more or less sleep than the common people.
7-9 hours of sleep per night may be good health, better sleep less than seven hours per night. There may be people who will need more or less sleep than others.
7-9 hours sleep per night may well be healthier than less than seven hours of sleep per night. There may be people who are born to more or less sleep than normal.