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It really depends on people. I know that my parents are around 55 and don't sleep more than 5-6 hours a day during the week and 7-8 during weekend and don't seem to need more.

Most of time it's between 7 to 9. What is important is to have a really repairing sleep. 4 hours of repairing sleep is better than 7:30 waking up several times and have pain issues.