Physiology

Our prehistoric brethren
knew it was time for bed
when the eldest pack member
began to yawn, setting off a string
of other yawns, as empathy
within a well-matched pack
demands we yawn as well.

Yawns cool our big, overheated
brains and set the stage for deep
sleep once again. We must receive
deep sleep in order to wash our brains
clean of daily waste, cerebro-spinal
fluid clearing away the sludge
our brains produce each day.