Out ancestors had huge cojones hunting mammoths.
In the '70s Psychology Today had an article on a study done on groups of college women living in the same group (dorms and sorority houses). It turns out that those groups tended to synchronize their lunar cycles.
If the same phenomenon existed within tribal units, ...
I guess there was one week a month when it was preferable to go out and risk your life attacking huge beasts with pointy things rather than remain in camp...