Not sure how this is any more or less arbitrary, or more helpful.
And at least you can plan "I need to be up in 6 hours" when you lay down instead of "I might have 6 and a half or 7 hours, but better plan on 6 just in case.
It's just as arbitrary, just based on a different, variable, irregular, and unpredictable (unless everyone gets degrees in astronomy). It may be predictable by the math, but no one wants to do any of that math every day.