[Terr_]

I assumed it was a kind of warning system: "Another member of the tribe detected impairment/fatigue and took measures to become more alert, perhaps you should raise alertness also."

[rhcom2]

Interesting you went in the opposite direction of my assumption, which was "another member is tired, perhaps we should all pack it it"

[Terr_]

AFAIK yawning is something our bodies do to fight or delay sleep, rather than to get to sleep faster.