Do you notice how when you get on a bus or into a car once it starts to move you start to get tired.
Many people fall asleep on these large moving machines.
Maybe it has to do with similar impressions that were made on us as fetuses in our mothers womb.
Imagine being back in the womb again though..But not Greek Mythology style (Remeber what happend to the Titans?). Well you can almost reproduce the environment without the amniotic fluid and the drugs that your mother took. Its called a FloatationTank aka Isolation Tank. Popularized by JohnCLilly. Even further, JCL asked the following question:
What happens if you remove the stimulation of the outside world?