Swiss brain researchers from the University of Geneva have found that rocking makes it easier to fall asleep.

For test purposes, twelve adult men between 22 and 38 years of age were asked to take two short 45-minute naps.
In one trial, the bed was fixed to the floor; in the other, it was rocked gently from side to side.

The researchers found that the rocking movement made the subjects fall asleep sooner.