[pbhjpbhj]

Assuming you were in a vehicle with ICE speed relates to vibration/noise which can, at the wrong pitch, cause drowsiness easily. This is why parents will take their children out in the car if a child is not sleeping well.

I find rough motorway surfaces in my current vehicle induce heavy drowsiness at motorway speed limits (slight reduced at marginally higher speeds when the pitch is higher).

[hippich]

This is interesting idea, never thought about it! thanks!