One voice in a crowd
At a crowded gathering a dozen conversations reach your ears at once, yet you follow one and the rest fade to a murmur. In 1953 the engineer Colin Cherry called this the cocktail party problem and built an experiment around it. His volunteers wore headphones that played one message into the left ear and a different one into the right, a method called dichotic listening. To hold their attention on one ear, they repeated its message aloud, word for word, as it arrived. This repeating is called shadowing.
Afterwards Cherry asked about the other ear. His listeners knew a voice had been there, and nearly always noticed when it changed from a man’s to a woman’s. But they could not report a single phrase it had said. They did not notice when it switched from English to German, and only a few heard anything odd when it was played backwards.
A filter with a leak
Why did so little of the second message get in? In 1958 the psychologist Donald Broadbent proposed a filter. Everything the ears pick up, he said, waits briefly at a narrow point, a bottleneck, and only one stream passes on to be understood. The filter chooses by sound alone: which ear, how loud, how high the voice. Meaning is worked out afterwards, and only for the stream that passed. Because the choice comes before meaning, the idea is called early selection.
One result did not fit. In 1959 Neville Moray slipped each listener’s own name into the ignored ear, and about a third of them noticed it. A filter that knew nothing of meaning could not have picked out a name. So the filter leaks. An ignored message is turned down, not switched off, and some of its meaning still gets through.
Two tasks, one queue
Listening is not the only thing with a limit. Try two demanding tasks at once and one of them suffers. What feels like doing both is mostly task switching: the mind is switching rapidly between the two, and every switch costs a little time and a few more mistakes.
The limit shows in the simplest test. Give someone two signals a fraction of a second apart, each needing its own response. The closer together they come, the later the second response is, because choosing a response is a job the mind does one at a time. The delay is called the psychological refractory period.
On the road this matters. In a 2001 experiment on simulated driving, people holding a phone conversation missed more than twice as many traffic signals. A hands-free phone was no better than a hand-held one, because what a conversation uses up is attention, not hands.
The gorilla nobody saw
In 1999 Daniel Simons and Christopher Chabris showed volunteers a short film of six people passing basketballs, three in white shirts and three in black. The task was to count the passes made by one team. Partway through, a person in a gorilla suit walked into the middle of the players, faced the camera, thumped their chest and walked off. They were on screen for nine seconds.
About half of the viewers never saw it. Some would not believe it had been there until the film was played again. The gorilla had been in front of their eyes like everything else in the scene, but their attention was on the count. Missing a plainly visible thing because attention is busy elsewhere is called inattentional blindness. It suggests that looking is not enough. We see what we attend to.
That answers the question you started with: A person in a gorilla suit walks into the middle of a ball game on film, stops, thumps their chest and walks off. Half of the people watching never see it. How can that be?
The stranger who changed
A year earlier, Simons and Daniel Levin had tried something bolder on a university campus. An experimenter stopped a passer-by and asked for directions. While the passer-by was explaining, two men carrying a door walked between them. Behind the door, the experimenter swapped places with a different man, who carried on the conversation as if nothing had happened. He was a different height, wore different clothes and had a different voice.
About half of the passers-by noticed nothing and went on giving directions to a new person. This is change blindness: failing to notice that something in front of you is no longer what it was a moment ago. The mind keeps far less of a scene than it feels as if it does. It holds on to what it is attending to and takes the rest on trust.
Awake, drowsy, asleep
Psychologists use the word consciousness for your awareness, from moment to moment, of your own thoughts and of the world around you. It is not simply on or off. It runs along a scale, from sharp alertness through relaxed drowsiness down to sleep.
The scale can be read from outside. In the 1920s the German doctor Hans Berger found that electrodes on the scalp pick up faint electrical rhythms from the brain, a recording now called the EEG. An alert, busy brain gives small, fast ripples called beta waves. Close your eyes and relax, and they slow into alpha waves. In the deepest sleep they become large and slow, and are called delta waves.
A recording like this shows a state: how awake a brain is. What awareness itself is, and what the soul is, lie beyond anything an EEG can show.
A clock that runs without the Sun
You do not get sleepy only because you are tired. A clock inside the body raises and lowers your alertness across each day, whatever you have been doing. It is called the circadian rhythm, from the Latin for “about a day”.
“About” is exact. People kept for weeks without daylight or clocks still sleep and wake in a regular cycle, but it drifts. Measured under controlled lighting, the clock’s own day lasts 24 hours and 11 minutes on average. In ordinary life the drift never builds up, because one signal, light, sets the clock back in step each day.
The rhythm has a shape. Alertness climbs through the morning, dips in the early afternoon, peaks in the evening and is lowest in the small hours. The siesta is taken in the afternoon dip, and the midday rest known in Muslim tradition as the qaylulah falls close to it.
Where the clock sits
The body’s master clock is a cluster of about twenty thousand nerve cells, no bigger than a grain of rice, deep in a part of the brain called the hypothalamus. Its name says where to find it. It lies just above the optic chiasm, the place where the nerves from the two eyes cross, so it is called the suprachiasmatic nucleus.
The position is no accident, because the clock is set through the eyes. The retina, the light-sensitive layer at the back of the eye, holds a small number of cells whose job is not to form pictures. Researchers identified them in 2002. They measure how bright the world is and report straight to the clock. They respond best to blue light, the colour of a daytime sky. Some people who are blind still have these cells, and their clocks still keep time with the Sun.
The darkness signal
How does a clock in the brain make the whole body ready for sleep? It sends out a hormone, a chemical messenger carried in the blood. As evening comes, the master clock signals a pea-sized gland in the middle of the brain, the pineal gland, and the gland begins to release melatonin. The level in the blood starts to rise about two hours before your usual bedtime, peaks in the middle of the night and falls away before morning. Melatonin does not knock you out. It tells the body that night has come.
Light at the wrong time breaks the message. Light in the evening holds melatonin back, and blue light does so most strongly, because the retina’s clock cells respond best to blue. A bright screen late at night therefore tells the clock that it is still day, and sleep tends to come later.
A night in rounds
Sleep is not one long blank. Through the night the brain runs the same sequence of stages again and again. One complete round takes about 90 minutes, so a night holds four to six of them.
The night begins with a few minutes of drowsy sleep, from which a small noise will wake you. This gives way to light sleep, the stage in which a young adult spends about half of the whole night. From there the sleeper sinks into deep sleep, when the brain’s waves are at their slowest and waking is hard. Then the brain climbs back up, and instead of waking it enters a different state altogether, called REM sleep, in which most vivid dreams come. After REM a new round begins, with only a moment of drowsy sleep or none.
Deep sleep first, dreams towards morning
The rounds of a night are not copies of one another. Deep sleep comes mostly in the first two or three. In this stage the scalp recording, the EEG, shows the largest and slowest waves the brain makes, called delta waves. Breathing and heartbeat are slow and even, and a sleeper shaken awake is groggy and confused for some minutes.
As the night goes on, the deep sleep thins out and almost disappears, and REM takes its place. The first REM period of the night may last ten minutes, and the last can run close to an hour. This is why an alarm so often cuts into a dream, and why a night cut short at the end loses mostly REM sleep.
An active brain in a still body
REM sleep was first described in 1953, in a Chicago laboratory where Eugene Aserinsky watched sleepers’ eyes darting about under their closed lids. The name stands for rapid eye movement. In REM the EEG looks almost like that of a brain wide awake, and people woken from it usually say they were in the middle of a vivid dream.
The body, meanwhile, is cut off. In REM the brain blocks its signals to the large muscles, so the arms and legs lie limp and cannot move. This paralysis is called atonia, and it keeps a sleeper from acting out the dream. When it fails, as it does in one rare disorder, sleepers kick, punch and leap out of bed while still asleep.
What the sleeping brain does with the day
Why spend a third of life asleep? One well-supported answer is memory. As long ago as 1924, an experiment had two students learn lists of nonsense syllables. They remembered far more of a list after hours of sleep than after the same number of hours awake.
A new memory is easily lost. Sleep helps to make it last, a process called consolidation. Recordings from sleeping rats show part of how it works. A structure deep in the brain called the hippocampus takes in the day’s new memories. In deep sleep the hippocampus replays the day’s patterns of activity, again and again and much faster than they first happened. The replay is thought to hand them on to the brain’s outer layer, the cortex, where they are kept for the long term.
Going without
What sleep is worth shows most plainly when it is taken away. In January 1964 a seventeen-year-old in San Diego, Randy Gardner, finished 11 days without sleep, a school science project watched in its last days by a sleep researcher from Stanford. By the end his speech was slurred and he lost the thread of what he was doing. Then he slept for nearly 15 hours and recovered.
Few people get anywhere near that, because the brain takes sleep by force. A badly short-slept person drops into microsleeps: lapses of a few seconds in which the brain stops taking in the world, sometimes with the eyes still open. Often the person never knows. At the wheel a few seconds is enough. A 1997 study found that people kept awake for 24 hours did as badly on a test of coordination as people over the legal alcohol limit for driving.
