Unaided by clock or sun, or any other medium known to our five senses, it keeps watch through the darkness.

At the exact moment it whispers "Time!" and we awake.

The work of an old riverside fellow I once talked with called him to be out of bed each morning half an hour before high tide.

He told me that never once had he overslept himself by a minute.

Latterly, he never even troubled to work out the tide for himself.