Not needing a lot of sleep.
This is a genuine, and unfair advantage, since it is almost entirely genetic, and not based around lifestyle or nutrition. Some people (Margaret Thatcher, Napoleon, etc) were able to function very effectively on just 4 hours of sleep, leaving them 20 hours in the day to be productive. Others need 8 hours just to feel normal, and that is 4 hours that they fall behind every single day, with no way of changing that.
It's easy nowadays to use all sorts of efficiency methods and systems to help you get more done in the day, but as you push this further you inevitably hit a limit where you are as productive as you can possibly be. Unnecessary work is delegated out to either people or technology, and you are left doing just that which requires your personal input. It's at that point that the extra 4 hours become a fundamental unfair advantage.
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