The Night Shift

There is a moment at dusk when the world seems to hand itself over. The last birds fall quiet, the diurnal animals bed down, and for a brief interval the land holds its breath. Then the night shift clocks on. Bats peel away from their roosts in sudden black streams. Owls unfold from the branches where they have waited out the day. Moths rise into the air, and foxes slip from their earths, and a whole hidden economy of life begins its work under cover of darkness.

Seeing Without Light

To live by night is to solve the problem of a world without sun, and evolution has found many answers. The most familiar is the eye rebuilt for darkness. Nocturnal mammals tend to have large eyes packed with rod cells, which detect dim light but not colour, and behind the retina many carry a mirror-like layer called the tapetum lucidum that bounces light back through the eye for a second chance at capture. It is this mirror that throws back the eerie glow when a torch beam catches a cat or a deer in the dark. A tarsier's eyes are each larger than its brain. A cat can see in roughly a sixth of the light a human needs.

But sight is only one route through the night. The bat abandons it almost entirely, firing out streams of high-pitched calls and reading the world from the echoes that return, building a sound-picture precise enough to pluck a single insect from the air in total darkness. The barn owl hunts largely by ear, its facial disc funnelling the faintest rustle of a mouse toward ears set slightly askew, so that a sound arriving a fraction earlier at one ear than the other tells the bird exactly where its prey is hiding beneath the grass. Snakes read the heat of warm bodies with pits in their faces. Many fish and some mammals feel their way through the dark by touch and by the subtle pressure of moving water.

Smell, too, comes into its own after dark. Scent lingers in the cool, still air of night far better than in the churn of a warm day, and many nocturnal hunters navigate a landscape of odour trails invisible to the eye. A hedgehog snuffling through a hedgerow, a badger following the scent of worms across a wet field, a moth tracking a female's perfume from a mile away: all are reading a world we cannot smell at all.

Why Go Dark

The question is not only how animals live by night but why so many choose to. For some, the night is a refuge. Smaller or more vulnerable creatures can forage under cover of darkness while the daytime predators sleep, trading the dangers of the dark for a quieter, safer window in which to feed. For others the night offers cooler air and less water lost to the heat, which is why so much desert life waits for sundown before it stirs. And for the predators, the darkness is simply where the prey has gone; the hunters follow their food into the night.

There is also competition to consider. The day and the night are, in a sense, two separate worlds sharing the same ground, and splitting the twenty-four hours between species is one way to avoid a direct contest for the same resources. The hawk owns the daylit sky; the owl inherits the dark. The same patch of woodland feeds two entirely different communities depending on whether the sun is up.

All of this makes the night a place of extraordinary richness, and increasingly a place under threat. Artificial light now bleeds across much of the inhabited world, and for creatures tuned to darkness it can be devastating, drawing moths to their deaths around lamps, disorienting migrating birds, and scrambling the ancient rhythms that tell an animal when to wake and when to sleep. The night shift has worked without interruption for millions of years. Whether it can go on doing so, in a world we are steadily flooding with light, is a question we have only recently begun to ask.