I have to sleep exactly 8 hours for it to be restful. Any less and I'm a groggy waste of space, any more and I just feel sluggish (though I'm at least able to get stuff accomplished).
Getting to sleep (and staying asleep) is definitely difficult for me because I'm such a light sleeper. I have to block the bottom edge of my bedroom door with a towel because if my roommate is awake the light from her bedroom will come in under the door and wake me up. Also, if the person in the apartment above me has their TV on I can just forget it.
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"Success is not the absence of failure, it is the persistence through failure."