Yawn
A yawn was a reflexive action that could indicate either fatigue or boredom.

In 2368, Data mimicked a tired 's yawn, to the boy's satisfaction. ()

In 2369, Q, disguised as a Bajoran waiter, dissuaded Julian Bashir from spending time with Vash by yawning to convince him he was tired too. () 

In 2370, just before initiating a dream program, Data yawned on purpose. ()

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Physiology
Sleep