Letters from Aldwin

Just over eleven thousand people in Britain wore a light sensor on the wrist for a week. Three years later, their hearts were scanned. The people whose nights stayed brighter had hearts a little changed: walls a little thicker, muscle a little stiffer, the squeeze a little smaller. The changes were subtle, but they were real, and they began low. Around three lux, which is less than moonlight, about as much as light under a closed door. The brighter the nights, the stronger the pattern, in a straight line.

Part of the reason seems to be sleep, which light quietly shortens. Night is when the heart stands down and gets worked on. Leave the room lit and it stays half on duty, bracing, rebuilding itself around the light.

That is the part I keep sitting with. We treat the dark as the empty hours, the hours a light is there to fill. For the body it is the opposite: the dark is the workshop, and most of the mending is done while you sleep.

So if you keep a light on, for company or for courage, that's human. You can still set the watch down for the night. Nothing about you gets lost in the dark. It is where you are tended, all night, by a muscle that never once forgets you.