How much sleep we need vs how much sleep we think we need

I wanted to follow up on another question that was asked about sleep. The answer stated:

“The amount of sleep we get is neutral. Whether we get 8 hours or 10, it doesn’t matter until we decide to think things like “I prefer to sleep no more than 8 hours…You can choose to believe that your body needs 10 hours of sleep.”

But the amount of sleep we actually need can’t all be based on our thoughts, right? I could believe that I only need 4 hours a night…or none at all…but my body would beg to differ!

I bring this up because I would like to sleep 6-7 hours per night but always feel better with 8-9. I’m exhausted with only 6 hours. Even after an adjustment period. It can’t all be in the mind, and there must be a biological limit, no?