I had to read these sentences in an article.
Most people need from 7 1/2 to 8 1/2 hours of sleep a night. You cannot save hours of sleep the way you save money in the bank. If you have only 5 hours of sleep for three nights, you don't need to sleep an extra 9 hours on the weekend.
I don't understand how to get 'an extra 9 hours'. Please explain it to me. I'm confused with the clause 'if you have only 5 hours of sleep for three nights'.