HEY, MR. SANDMAN
A REAL SUMMER FOR me includes swifts. They’re like swallows but much bigger and much, much faster. Uttering their piercing calls, they race between tall city buildings, hunting for insects or just for fun. Unlike many other birds, swifts spend almost their whole lives in the air. They are so extremely adapted to life above the earth that their legs have atrophied and their tiny feet are good only for clinging onto things. Of course, they also need to breed, and their nests, which they build in cliffs or in cracks in walls, are constructed so they can fly in and out of them with ease. Apart from the time they spend sitting on nests, the birds satisfy all their other requirements on the wing. Even mating often takes place high up in the air, where swifts, like us, abandon themselves to the moment. And having a male clinging to the female’s back does nothing for the birds’ flying prowess, so mating pairs often end up spiraling down at breakneck speed, and they have to separate in time to avoid being smashed to smithereens.
But I wanted to introduce you to the swift because of another bodily function: sleep. Most life forms (even trees) need to sleep, and birds land in a sheltered place to do so. Our chickens, for example, take themselves back to the hen house at dusk like good little chickens, tramp up the ramp, and settle on their perch, where they snuggle up side by side. They don’t have to worry about falling off, because, as with most birds, their tendons shorten when they sit and their toes curl automatically. This means our chickens can hold on tight without expending any energy. Like all birds, chickens dream. When they do, there’s a danger they might move around, just as we do, as their nightly movies play. But if they were to move, they might fall from their perch or, for birds living in the wild, out of their tree. And that’s why the muscles used for movement are deactivated the moment a bird nods off, so it can spend the night peacefully with its head tucked underneath its wing.
And what about swifts? They never perch, and they spend not a second longer than they have to on the ground or on the nest. If they want to sleep, they do so while airborne. That is highly risky, of course, because sleeping birds aren’t in total control of their actions. And so they spiral upward a mile or more to increase the distance between themselves and the ground. Then they begin to glide downward, tracing a wide circle that slows their descent. Finally, they are free to doze for a few moments. They don’t have time for anything more, because they need to be wide awake again before the first rooftops loom dangerously close and their situation becomes precarious.
Is this brief shut-eye sufficient for the birds to get any rest? Definitely, because although sleep allows all species to exclude or reduce outside influences so the brain can run its internal processes undisturbed, sleep is a little different for every species. The different phases of our sleep with their varied depths show that even human sleep is not a uniform affair. Our horses, for example, don’t need much in the way of really deep sleep. Often just a few minutes are enough, which they take while lying down on their sides looking as though they’ve been shot. They’re so deep in dreamland that they are indeed dead to the world, and their legs twitch as though they were galloping over an imaginary prairie. Other than that, they stay on their feet and doze away a few hours of each day just like the airborne swifts.
It’s obvious that, like us, animals sleep. Even tiny fruit flies need to sleep, and when they’re asleep, they twitch their legs just like horses. The really exciting question is how they sleep and what they dream about. Our nightly mental excursions take place during the so-called REM phase of sleep. REM stands for “rapid eye movement.” In this phase, our eyes move under our eyelids, and if you wake a person in REM sleep, they can almost always remember what they were dreaming about. Many species of animals experience similar nightly eye movements, and the larger their brain in relationship to their body size, the more they have. Because animals can’t talk to us, we have to look for other clues to understand what’s going on inside their heads. Researchers at the Massachusetts Institute of Technology in Boston looked at rats. They measured the rats’ brainwaves while they were in a maze busily searching for food. Then they compared these readings with instrument displays while the rodents were sleeping. The similarities were so clear that, using the data they had gathered, the researchers could even tell where in the labyrinth the sleeping rats were in their dream.78
Experiments with cats in 1967 also yielded evidence for dreaming, although in this case the evidence was indirect. The scientist Michel Jouvet from the University of Lyon stopped cats from relaxing their muscles while they slept. Normally the body, including the human body, shuts off voluntary muscle movement to prevent us from thrashing about wildly in our dreams or walking around our bedrooms with our eyes closed. This shutoff mechanism is only necessary in dream sleep. When the mechanism is deactivated, you can observe what the test subject is experiencing in its dreams. Jouvet observed cats in this state arching their backs, hissing, or running around, all while in a deep sleep. Science accepts this as proof that cats dream.79
But what does it look like if we distance ourselves from our branch of the animal kingdom, leave the mammals, and consider insects instead? Could something similar play out inside such tiny heads? Can the relatively small number of cells in a fly’s brain also produce pictures during sleep? Today there are indeed indicators that these tiny clumps of cells can do more than we ever gave them credit for. As I just mentioned, fruit flies twitch their legs just before they fall asleep, and their brain is particularly active while they are sleeping—another parallel with mammals. Does this mean that fruit flies dream? Their physical reactions suggest that they do, but so far we can only guess what images might be lighting up inside their little heads. Pictures of mushy fruit, perhaps?80