system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If 'rest' is such a critical biological necessity, why do the heart and lungs (made of the same material as the rest of the body) not need it?
gpt
When an animal sleeps, (delta anyway, there is a bit more action during REM sleep) you'll notice a general slowing of the breathing and heart rate.  When a person is lying down, it also reduces stress on the heart, since it doesn't have to fight as much with gravity to keep blood moving to the head.

That said, 'rest' as in sleep doesn't really apply to any autonomous bodily functions.  Digestion continues, liver and kidneys keep functioning, and cellular respiration carries on at a more or less normal rate.

The biggest functions of 'rest' - aside from resting after exertion, when the body gets an opportunity to clean up nasty byproducts like lactic acid - seem to have to do with the brain.