There are reasons why we sleep. Physiologically, our body need some quiet time to do maintenance (we don't fix a car when it is running). In our sleep, only essential functions are running. The body can work on other parts.

How long do we "need" to sleep? A simple answer is "as long as it takes to revitalize the part that takes the longest time. Our body in advanced years can take longer to return to normal function (due to blockage, imbalances, residual deposits, ...).

How do we know whether we have had enough sleep? Our body can tell us in many ways. Yawning, dozing off in meetings, feeling tired,...

We can take medicines for this. But we can't use the hours 4/5/6... (I think) diet, exercises and good sleep are best. ;-)