Where we go when we fall asleep

We say we "fall" asleep, as if into nothing. But we do not fall into nothing: we depart. Falling asleep is not a switch that flips, it is a passage with stages. And the first stretch has a name: the hypnagogic state, the threshold where we are no longer awake and not yet asleep.

There, at the edge of sleep, the day comes back. The images of what we did rise on their own. Robert Stickgold showed it with a game: people who had spent hours fitting Tetris blocks saw them falling behind their eyelids as they dozed off. And so did amnesic patients who could not remember playing at all: the brain replays the day before conscious memory can even tell it (Stickgold and colleagues, 2000). At the door of sleep, the day just spent comes knocking.

And it is not wasted time: while we sleep, the brain files. Memory is reorganised, moved, consolidated; the hippocampus replays to the cortex what it lived, and what matters is kept (Rasch and Born, 2013). We go to sleep also to sort out where we have been.

Because a world, in sleep, is built. Dreaming does not live in REM sleep alone: the brain generates an experience, a place, and it lights up in a posterior region when the dream is there (Siclari and colleagues, 2017). We do not go into nothing: we go somewhere the brain makes.

So where do we go when we fall asleep? We go inward, into a workshop: the day is replayed, sorted and remade into a world. Not a switching off, but a departure, with its stages.

And the road? On the road that departure asks no permission. We believe falling asleep is instant, a line we will not cross by accident. It is not: it is a slide in stages, and at the wheel the first stage - the hypnagogic image that is no longer the road, the thought that drifts off - is already the leaving. The microsleep is this same passage, stolen. The danger is believing the door is far when we are already leaning on it.

Before setting off on the road, remember there is another journey that can begin on its own.