Why Do Trains Make Us Sleepy?

the polar express and nighttime transit as the ideal dream scene

The whole of The Polar Express feels like a dream. It’s snowing, but the snow is soft, like a white mist. It’s windy, but not enough to make the train sway on its tracks. The children sit in pajamas on the train; outside, the night is soft, clouds are mixed into snowflakes mixed into bubbles of steam from the train’s engine. The characters have distinctly non-human faces; their eyes are too far apart, their gazes are glassy and unfocused; their skin is waxy. Alan Silvestri’s orchestration, taking from Josh Groban’s “Believe,” plays in the background. A wordless choir sings sustained vowel sounds, hollow and ghostly, like angels through a fog. Bells twinkle and the string instruments lay a warm undertone, swelling and receding with the choir’s vowels. The celestra, known for sounding like what we imagine snowfall should sound like, shimmers while the French horn rounds in and then back out again. It sounds like a good-night hymn gushing ashore. When the credits roll at the end, Groban’s lyrics finally come in: “Children sleeping / Snow is softly falling / Dreams are calling / Like bells in the distance…Trains move quickly to their journey’s end / Destinations are where we begin again / Ships go sailing far across the sea / Trusting starlight to get where they need to be.”

The movie looks and sounds like a dream, dark and soft and hazy, almost real but distinctly not, set on a sleepy steam engine that puffs through the night as if it were singing a lullaby. The train chugs along toward the North Pole, exhaling clouds of steam up toward the stars. A series of rods and pistons turn the wheels with a steady rhythm. The wheels spin in discretized circles, each linear push of the piston translating into another full circle. Inside, the train is warm. The seats are a deep red, as is the rug laid across the middle aisle. The walls are a toasted wood color. At the back of the car is a green door with a circular glass window, like the inverse of a holiday wreath. The children wear patterned pajamas, nightgowns and robes and cotton long-sleeves. The pistons extend, the wheels clink into another circle, a new breath of steam mixes into the night.

When I have trouble falling asleep, which is not often, I like to imagine that I am in a submarine. It’s me and I’m wearing a soft red sweater, and sometimes I have a little brown dog with me. I have a wooly blanket too, and there is a small circular window on the wall of the submarine so that I can look outside. Outside, it is quiet, blurry with bubbles and the occasional small fish. The submarine is small. There is not much furniture aside from the armchair on which I sit. The submarine moves along slowly, bobbling steadily up and down and forward through the ocean, with me inside. Usually, by this point, I have fallen asleep.

In 2011, a group of scientists at the University of Geneva in Switzerland studied how rocking—in a steady rhythm much like that of moving trains or submarines—affects sleep. Participants in the study took 45-minute naps, once on stationary beds and once on rocking beds that swayed gently at 0.25 Hz (one full swing every four seconds). They fell asleep faster on the rocking beds and spent more time in deep sleep than they did on the stationary beds. In 2019, a similar study found the same results for a full night of sleep. That same year, another team led by Paul Franken at the University of Lausanne in Switzerland aimed to figure out why: why does rocking help us sleep? Franken’s team found that it had to do with otoliths, the tiny calcium carbonate crystals embedded in our inner ears. Otoliths sit inside the gel-like membrane that surrounds hair cells in our ears. As we tilt or move our heads, the otoliths weigh down our hair cells, bending them and sending signals to our brian to indicate that we’re moving. Franken’s team created a group of mutant mice who lacked otoliths. They found that while normal mice fell asleep faster and slept more deeply when rocked, the mutants experienced no sleep benefit. The little crystals inside our inner ears, they concluded, hold the key that connects rocking to sleeping; without them, the connection disappears.

It is possible, I think, that the connection between rocking and sleeping, and similarly between transit and sleeping, evolved over time. In 1980, scientists JoAnne Brewster and Michael Leon at McMaster University in Canada studied rat pups’ responses to being picked up and carried by their mothers. When their necks were taken in their mothers’ mouths, rat pups assumed a characteristic change in posture. They stopped squirming, their forelegs folded inward, their hindlegs flexed outward, their tails curled in between their legs and toward their bellies. They became compact and quiet as their mothers carried them, almost as if they were settling in for a good night’s sleep. Brewster and Leon called this the “transport response.” Other scientists began noticing it in other species too, including cats and primates. In 2013, a team led by Kumi Kuroda at the RIKEN Brain Science Institute in Japan observed a similar response in human infants. When carried by their walking mothers, infants stopped squirming and crying. Within seconds, their heart rates dropped, much more quickly than they did when the infants were held by their sitting mothers. As their mothers walked around, the babies fell into a calm and quiet state, one that if sustained would likely lead them straight into sleep.

The transport response has evolutionary benefits. A calm and compact baby is easier to carry than a rowdy one. When a mother moves her baby away from a predator, or to a new den or across harsh terrain, the baby who struggles and cries is much more likely to attract danger, and to die, than the baby who folds in his legs and hangs limply while his mother carries him. Biologically, the transport response relies on both tactile and proprioceptive signals; to launch the response, babies must both feel their mother’s touch and feel themselves moving through space. These signals are processed by the cerebellum in the brain. They then pass on to the vagus nerve, which tells the heart rate to slow, the muscles to relax, and the cries to quiet. This response is said to fade when babies reach four months of age, and it operates independently of the otoliths in the ears that affect adult rocking responses; so it is unlikely that the biological mechanisms of the transport response precisely carry over into adults. Still, the transport response suggests that there may be evolutionary pressures guiding the link between transit and sleep. In adults, the otoliths provide this link. The inner ear contacts the brainstem, which then contacts the vagus nerve. Signs of steady movement, like those from a rumbling steam train or a bobbling submarine, transfer from the otoliths to the brainstem to the vagus nerve, slowing our heart rates and setting us up to fall asleep.

At the end of The Polar Express, the main character gets dropped off in front of his house. He stands in the doorway as the train conductor waves good-bye and the train puffs away. Inside, his house is dark and quiet. In the living room, milk and cookies are laid out for Santa Claus. He goes upstairs and presumably, he goes to sleep. The next time we see him, he is being shaken awake by his sister. “Santa’s been here!” she exclaims. The two rush downstairs and, for a moment, we wonder if the Polar Express and the trip to the North Pole was really all a dream. The two children sit in front of the tree and the boy unwraps a small gift from Santa: the Christmas bell that he dropped in the North Pole last night. It turns out that everything was not a dream, although it certainly felt like one. Groban’s lyrics come in as the end credits roll. “Trains move quickly to their journey’s end / Destinations are where we begin again / Ships go sailing far across the sea / Trusting starlight to get where they need to be.” Like any good passenger, the silent ships trust that they will get where they need to be. The train clinks forward, the wheels turn against the tracks. The children sit in soft pajamas on rows of red cushions. Our otoliths register the motion, our bodies process the steady rhythm, our heart rates slow, and we begin to fall asleep. “Destinations,” Groban sings, “are where we begin again.” Anything before then—anything in the space between departure and destination—is reserved for the dream.