Why does the world turn grey and blurry when the lights go low?

In dim light your cone cells, which carry colour and fine detail, no longer get enough light to respond, and a second set of detectors called rods takes over. Rods can't tell colours apart and they blur fast changes, so the world turns grey, soft and slow. You haven't lost your sight, you've changed sensors.

Your retina holds about 92 million rods and only about 4.6 million cones. Rods are the more sensitive. A single rod can respond to one photon, and it is about 100 times more sensitive to a single photon than a cone, which needs tens to hundreds. Many rods also feed into a single interneuron that gathers and amplifies their signals. The price is that rods say little about colour, and they add up light over roughly 100 milliseconds, so quick changes smear.

That explains the quirks of night. At twilight, colours drain away, and for a while the blue-greens hang on longest because the rods are most sensitive there. Walk from daylight into a dark cinema and you stumble at first. Adaptation in the rods is about 80 percent done after thirty minutes.

Astronomers use a neat trick. The fovea, the pit at the centre of your gaze, has no rods, so a faint star can vanish when you stare straight at it. Looking slightly to the side, where rods are denser, brings it back. And because rods are insensitive to deep red, many people use red torches to keep their night vision.

Many animals do better in the dark. Cats and others have a mirror layer behind the retina, the tapetum lucidum, that sends light back through for a second try. It is why their eyes glow in a flashlight beam. Humans, being diurnal, lack it.