At what level of sleep does muscle growth start to take a hit?

Sorry, I couldn't find a place where this question would really fit, so this is where I ended up.

I get about 6.5-7 hours of sleep 2-3 nights a week, and then 8 for the rest because of school.

At what point ( 7, 6, 5, etc.) does muscle growth start to take a hit?

Thanks.

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