Question

What contributes to the lethargic feeling when riding in a car for long periods of time? Could inertia play into it? Looking for the scientific explanation of how movement affects the body.

Answer 1

Exactly the same reason babies' cradles are built on curved runners, for rocking, and why we put the baby in a swing or rock the baby to sleep in our arms.  We are hard-wired to become soothed and relaxed by gentle repetitive motion.

But that still isn't an explanation of "Why".  My own understanding is that this is a carry-over from the earliest stages of our brain formation, when we were suspended in a pool of warm fluid in utero and our mother walked around during the normal activity of her normal day.

In my own personal case, I could never be an aircraft pilot.  As soon as my seat-belt is buckled and the airplane starts bobbing up and down, even while still on the ground, I am out cold.  I seldom remember take-off.  I have been on commercial business flights, that lasted 18 hours and I slept through 16 of them.  Turbulence in flight is even better.  If I happen to be awake, flying into a storm will knock me out in seconds.