What is the minimal amount of sleep you can have without harming yourself?

i take online classes, but they are very strict about how much time you have to complete assignments. i also work from 5- 11 at night. i am trying to factor in exercise with school, but it takes me at least 2 hours to exercise. i would really like to get healthy. any suggestions?
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Thank you to everyone for your responses.
What is the minimal amount of sleep you can have without harming yourself?
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