The periodicity of the boat on waves is calming in a similar way to rocking on a cradle. Something about the consistency of the motion offers security. I’d gues

The periodicity of the boat on waves is calming in a similar way to rocking on a cradle. Something about the consistency of the motion offers security. I’d guess it serves as an additional equilibrioceptive cue to the sleeping brain that’s sensitive to changes in the environment (e.g. an intruder).