A term used to describe the amount of time between when a person wakes up and when they go to sleep. It it useful when pulling all-nighters, or if your schedule is shifted and you are frequently awake late at night. It is more accurate to use this term if you frequently get your days mixed up, as it is often confusing to describe your "morning" or your "night" when you have witnessed many of them in a single sleep unit.
I haven't slept in 36 hours! I've written an 8-page paper, eaten 2 breakfasts, 2 lunches, and seen a movie, all in the same sleep unit!