In the first days, there was no watch. People slept when they were tired, all of them at once, and woke to find the world as they had left it. This went on for a long time.
Then one season there was a night when too many slept deeply and something was lost. A small loss, nothing anyone could name. A sound that had always been in the air that was no longer in the air. A path through the forest that still existed but felt, from then on, slightly different from how it had felt before. People noticed, without knowing how they noticed. They discussed it without having language for it.
After that, someone always stayed awake.
The practice was not invented. It was not decreed. It simply began, the way certain necessary things begin, through an arrangement arrived at without discussion. One person at a time. The watch rotated. When your turn was done, you woke the next and said the words that had come to mean: it is still here, and it is yours now, and you can go back to sleep.
Over many generations the original reason was forgotten. The watch was kept for new reasons: predators, fire, the practical needs of sleeping communities. These were real reasons and they were enough. The old reason did not need to be remembered because it had already been addressed. Someone was always watching. The problem had been solved before it was understood.
In the stories told much later, the watcher at the edge of the fire is not a hero. They are not exceptional. They are tired, and cold, and waiting for the person who will come to replace them. When the replacement comes, the watcher stands, says the necessary words, and goes to sleep. By morning they do not remember the watch. It does not need to be remembered. It only needed to happen.
Go back to sleep. Someone else will take it from here.