Ana Guerrero Braña, a psychologist at the European Institute of Positive Psychology, explains something most people know intuitively but have never connected to hormones: an untidy bedroom raises your cortisol. "A tidy bedroom reduces cognitive load, the brain does not have to process visual stimuli, and that lowers cortisol," she says.

The logic is simple. Every object that is not in its place is read by the brain as an unfinished task. The open drawer, the shirt on the chair, the box that has been sitting there for three weeks - none of that is decor, it is a to-do list lying in your field of vision while you are trying to fall asleep.

"Untidy environments contribute to a rise in cortisol, the stress hormone," Guerrero adds. And cortisol and sleep are in direct conflict: the more of one, the less of the other.

What the psychologist offers as a solution costs nothing at all. Closing the drawers, laying out tomorrow's clothes, clearing the bedside table - a few minutes of ritual that tells the brain the day is closed. Not meditation, not an app, not a supplement.

And this is exactly where the story gets more interesting than it looks. The sleep industry sells mattresses, "warm light" lamps, melatonin drops and devices that measure your sleep down to the second. Very little of it tells you what the psychologist said for free: that the problem may not be in your body, but in what is sitting in front of your eyes while you lie there.

Does this apply to everyone? No. There is insomnia that the tidiest room in the world will not fix. But if you are at the start of the list of possible causes, this one is the cheapest to test - and the only one nobody can charge you for.