That's a terrible null hypothesis, as there is no reproducible, controllable test for it, other than a time machine.
A better one might be "people exposed to pre-industrial revolution photoperiods (<12h per day) will settle into a biphasic sleep pattern".
This was empirically tested with a small N (https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2869.1992...). At least it's testable.
