It is 13 hours. the previous answer was 17, but that is WRONG
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8 hours
9 * * * * * Actually, it is 8 hours.
There are (24k + 9) hours where k is an integer.
13 hours. 8 pm to 12 am (midnight) is 4 hours. 12 am (midnight) to 9 am is 9 hours. 4 + 9 = 13 -- 13 hours of sleep (if you fall asleep exactly at 8 pm and awaken exactly at 9 am, have not awoken in your sleep, and have not changed timezones)
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