#151 - The Geography of Sleep

0:02Matt Walker: Hi there, it's Matt here and welcome back to the podcast. Let's begin with an object you probably never pay any mind to anymore. There is almost certainly a clock nearby. It could be on the wall, the oven, or the bottom right of your monitor. The most impartial object in your home, you might say. Just one job, no opinions. So let me ask you something you probably only considered when you were six. When that clock says 7am, 7am, 7am, by whose standard? It turns out that number is not as simple a fact as it seems. It's a choice, a border on a map, drawn by a committee, and then flattened over a massive area of the globe.

0:43Matt Walker: And deep under your skin, your body has its own clock, a clock governed by the sky, the season, and your location on this planet. This is a story about the space between those two clocks, the wall clock and the clock in your cells. Your place on the earth dictates your nights in ways the wall clock is trying very hard to ignore. Here's how it's ignoring you. Before I explain why your sleep might vary depending on where you live, I first need to explain where we do in fact live. Humans are not evenly spread across the globe. 88 % of humans live north of the equator. 50 % of humans live between 20 and 40 degrees north latitude.

1:27Matt Walker: That's a thin slice across the Mediterranean, the US, South, and Northern India. We are predominantly a northern hemisphere species crowding around the globe's waistline. We also live low and we live flat. Half of humanity lives at an altitude of less than 200 meters. Overall, across billions of people, a small fraction of people actually live in the mountains. The mountain village and the Arctic outpost capture our imaginations because they are so unusual. So when we discuss the geography of sleep, we are talking about this dense, low northern band and the two metrics that quietly dictate your sleep.

2:08Matt Walker: First, latitude, which determines how dramatically your daylight changes across the year. Second, longitude, which determines whether or not the clock on your wall matches your internal time. Let's start with latitude. Here's something you've always sensed, though you may not have thought of it in these terms. You know that the 7 a.m. light is very different in January from what it is in July, even though both show 7 a.m. on the wall clock. We know that across the seasons, there is a varying amount of daytime light and inversely nighttime darkness. So why does the changing of how much daylight we receive affect us?

2:48Matt Walker: It's because you're actually keeping another kind of clock and it isn't a clock at all. It's more like a calendar and it's written by a hormone. Here's how it works. Imagine melatonin, the hormone the brain releases when it thinks it's nighttime as the tide of the night coming in. In winter, there are many hours of darkness. The tide comes in early and stays in late. In summer, there are few hours of darkness. The tide is only in for a short stretch. If you put people on a winter-like length of night, their melatonin signal stretches out to match the darkness. If you put them on a summer-like length of night, it shrinks back to fit.

3:28Matt Walker: What moves with the seasons is the length of it, not the strength. This is the same mechanism that tells a deer when it's time to grow its winter a coat, we share this clockwork and the hormones that go with it with other mammals. So why don't we notice? Because of light. Take a small group of people camping, sleeping only by daylight and firelight for a week, and the seasonal clock resets. Their biological night starts some two and a half hours earlier in winter than it does when they are living under electric light at home. In summer, their melatonin profile fits nicely with the day length. Your biological night has been hidden from you.

4:07Matt Walker: The clock on your wall is doing its job. It keeps you on a steady schedule all year round. But the wall clock is lying. When you expose yourself to the natural cycle, the season appears. When researchers measured the sleep patterns of people living in pre-industrial societies in Namibia and in Bolivia, they found they get around an hour more sleep in the winter than in the summer, going to bed earlier when the nights are longer. When they measure sleep in natural conditions, it is clearly seasonal. So the question is, how much of it lasts when you go indoors and flip a switch? The truth is, some and less the closer you come to the equator.

4:46Matt Walker: Track sleep logs in Norway at 69 degrees north and Ghana at 5 degrees north, and the seasonal difference in sleep timing only really appears at the site further north. At the equator, where day lengths remain more or less constant throughout the year, there is hardly anything to react to. When you aggregate the recent data, contemporary humans still sleep slightly more during the darker part of the year, but by 20 minutes, whereas our ancestors slept almost an extra hour. Latitude still governs your sleep. It's just no longer hours, but minutes. This is the part that I've found the hardest to accept, the best surprise in the tale.

5:29Matt Walker: One city on earth experiences some of the most dramatic swings in daylight, and its residents barely change their sleeping habits. That city is Tromsø in northern Norway, deep within the Arctic Circle. During the summer, the sun stays above the horizon for weeks on end. During the winter, the sun remains below the horizon for weeks on end. If you were going to see latitude dictate your sleep, then this would be the place to see it. But poll roughly 21 ,000 of its residents aged 40 and over across different times of the year, and their sleep duration remains remarkably stable. Monitor other modern-day populations, and most of them do still show that small seasonal swing.

6:16Matt Walker: Tromsu shows none of that swing. How can that be, you might ask? Here's the truest point in all of this, and it's one that always gives me a smile. The physiology that I have outlined is absolutely true, and a person living beneath some of the most extreme skies in the known world sleeps the same number of hours every night of the year. The physiology hasn't stopped working. It's been muted. What's muting it is artificial light and you can see it in action. Look at two groups of indigenous people in Argentina who are similar in all respects except one. One group enjoys artificial light in the home.

6:58Matt Walker: The other group lives by natural light alone. The group with electric light has shorter sleep durations and falls asleep later at night. The same division shows up in groups in the Amazon. And this is also why the equator tells us the same thing, from the opposite side of the planet. Because where day lengths barely vary during the year, your sleep is dictated not necessarily by season, but simply by whether you have artificial light. The Arctic and the equator, both at the extremes of the latitudinal spectrum, come to the same conclusion. It's the light we generate that defines the modern night more than the sky.

7:38Matt Walker: One latitude effect, however, does not disappear. This has to do with the timing of sleep rather than duration. Get the chronotype information from tens of thousands of individuals in 28 countries and how much of a morning lark someone is correlates with their latitude. You can imagine that this could be the result of the time zone. So let us isolate this effect. Use nearly 13 ,000 people from within one time zone going from the equator to 32 degrees south. The higher the latitude, the more evening-leaning the people. Without the clock, the latitude still shapes the circadian cycle, even if it no longer affects its extent.

8:20Matt Walker: But the most telling evidence of humans as a seasonal species is not to be found in sleep. It lies in the birth month. Human births are not uniform throughout the year. There is a seasonal pattern to the number of births, with some months having more births and some having fewer. The timing of this pattern varies. From historical birth records, there is an annual reproductive cycle related to the photo period and the climate that has weakened over time as electric lighting and modern life have eliminated seasonal influences on human behavior. This pattern was strong and ancient enough to be a biological signature, just like other animals.

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13:02Matt Walker: Looking at 78 years of U.S. birth records, the birth peak moves with latitude. The northernmost states have their birth peaks in spring and summer, while the southernmost have them in autumn. The amplitude of the seasonal variation is larger in the south than in the north. The month of the birth peak tracks the local swing in day length, though the demographers are honest. That light, temperature, and plain human custom are all tangled up in it. Furthermore, the pattern can also shape another important seasonal cycle, childhood infectious disease. These two cycles, birth and childhood epidemics, work together, and the season of birth still carries something of the local light environment.

13:50Matt Walker: We like to think that we've overcome the seasons, but our bodies haven't quite finished forgetting them. The place where the signal of the seasons comes roaring back loudest is the one where very few of us actually live, at the poles. People spending the winter at Antarctic research stations, where there are months of continuous darkness, suffer from drifting circadian rhythms. And how do they recover? By a pulse of bright artificial light in the morning, which resets their internal clocks back on track. This is perhaps the most convincing evidence that our ancestral relationship with the seasons was never really lost.

14:28Matt Walker: It simply needs an extreme enough environment and a lack of electric light to make itself known. That's the case with latitude, how your wall clock quietly lies to you about the season. Longitude is even more bizarre. It's a season invented by your clock. So what time is it? When the sun is actually directly overhead, the answer depends on whether you're at the east or the west edge of your time zone. Solar time advances roughly four minutes per degree of longitude, but your clock is static. It stays fixed throughout the entire zone, then jumps an entire hour at once at a line drawn on a map. Imagine a large area deciding to call one moment noon when the sun, which didn't sign onto the plan, is happily keeping its own time.

15:23Matt Walker: At the eastern edge of the time zone, the sun and the clock are roughly in sync. But at the western edge of the zone, it can be almost an hour before the sun reaches its zenith. Your alarm runs on clock time, Your body runs on solar time. And the distance between the two depends, quite literally, on how far west you live from the center of your time zone. And there is a cost to this gap that you can measure in sleep. Consider people living on either side of a time zone boundary in the US, basically identical groups of people in the same country. The only real difference is that one group sits at the western end of its time zone and gets an additional hour of evening light.

16:09Matt Walker: Those people sleep around 19 fewer minutes each night and more often report not getting enough sleep. It's 19 minutes and that really doesn't seem like much, but it is every night for an entire lifetime. It comes at no extra cost to you other than what side of the demarcation line your house happens to be built on. And the late sun drags the body clock along with your bedtime, but not work and school, which start at the same fixed time. The sun lets you sleep late. The alarm does not care. Do the same exercise for a country with a single time zone and a single national clock. India is the prime example.

16:50Matt Walker: It covers close to two hours worth of solar time from east to west, but the whole of it falls within a single time zone. You can see the consequences of this playing out in the test scores and years of education of its children. Those children who live in the parts of the country where the sun sets an hour later also go to bed around 20 minutes later, yet they must still attend school the next morning. As a result, they miss out on roughly half an hour of sleep a day. And this lack of sleep is reflected in both their test scores and how many years of education they ultimately complete. This is the cost of having the same clock used across a territory that spans multiple longitudes measured in the lost sleep of children.

17:38Matt Walker: One final note, China has the world's most extreme example of this problem. In the entire massive country spanning five geographical time zones, the clocks still run on a single Beijing time. So in the far west of China, the morning can commence while the sky outside remains pitch dark. There's a name for this milder everyday version of the phenomenon, the kind we're all living with, social jet lag. Not the jet lag of a long flight, but the chronic low-grade mismatch between the timing of the sun and the timing of our clocks and schedules. Social jet lag is not something we get for free. Take a pool of hundreds of thousands of people and you find the wider the social jet lag of an individual, the higher their chances of being overweight and, in smaller studies, of metabolic disease.

18:32Matt Walker: Of course, these are correlations. They do not prove that social jet lag is the causal factor, but they all point in the end in one direction and it's not a happy The most contrived and invented season of all is the one we manufacture for ourselves by the edict of government twice a year. Daylight saving time. In standard time, sleep on your days off flows with the advance of the dawn. The body remains coupled with the sun as it should be. With daylight saving, this sink is broken. The body never quite gets used to the new hour. It's just a bit out of step for months on end. And in the spring, when we lose an hour in a single night, there's a real physiological effect.

19:19Matt Walker: Add up the research and the number of heart attacks increases by about 4 or 5 % in the days immediately after the spring clock shift. And significantly, only in the spring, not in the fall, when we gain an hour. One hour written in the law, echoing right into the heart. There is one more coordinate I would like to mention quickly, because it relates to geography as clearly as the other two. And because I've covered this in another podcast already, I'll restrict myself to the geographical aspect of it and no more. Temperature. Your brain allows you into deep, dreamy sleep only within a narrow band of temperatures.

19:59Matt Walker: And if you move the bedroom too far in either direction from comfortable, then the quality of your sleep goes down, both hot and cold affected. And this has geographical significance simply because heat is not distributed equally on the planet. Connect nighttime temperatures to sleep in 7 million nights across 68 countries and above approximately 30 degrees Celsius, some 14 minutes of sleep quietly disappear. And the biggest losses are in the warmest and poorest parts of the world. And the further you get from the equator, the cooler your nights become. And if you're too hot, it eats into your sleep.

20:38Matt Walker: Let me sum up the map, because underlying all the details is a battle between two clocks. The one inside your body was set by your latitude, by the length of your melatonin night, the timing of your sleep, even the season of your birth. The clock on the wall was set by us, with the light bulbs that blanketed out the seasons, and with the lines of longitude that forced a single imposed hour onto a thousand miles of land. And there's a silver lining here, a reason why you shouldn't feel helpless about this. So much of what's shaping your geographical night now happens indoors. And indoors is something you can change.

21:20Matt Walker: So here's what your coordinates suggest. A list of practical steps to take. Take in light in the morning, especially during the winter months in high latitudes, when natural light cues are at their lowest when you most need them. Morning light is a well-established therapy for winter depression, with benefits on a par with antidepressant medication, and dawn-simulating light isn't far behind. Figure out where in your time zone you are, and if you're on the western late sunset end, intentionally guard your sleep rather than allowing your late light evenings to inadvertently delay your bedtime. Treat the spring clock change for what it is to your physiology, a one-hour shift across time zones, and protect your sleep during that time.

22:10Matt Walker: Keep your sleeping space on the cooler side of comfortable, and keep an eye on your own personal jet lag, the distance between your biological clock and your social clock and shorten that distance whenever you can. Then one last thing, just for fun, since there's nothing to do about it anyway, the season in which you were born leaves a barely detectable mark on the clock you carry around for the rest of your life. Those born in autumn or winter, as the days are shortening, tend to be slight morning larks, an imprint you can still sense years later. There's nothing you can do about this, but it's a lovely reminder, perhaps, that we're light creatures from the very first day of our lives.

22:53Matt Walker: So have another look at that clock on the wall. It's not telling you the time. It's telling everyone across a thousand miles of land the same neat little fable, a tale that your body believes only partially. And that may be up to an hour off, depending on where you're standing. That wasn't the real clock. Your real clock is the one you came with, continuing to tell its own quiet time. Thank you so much for listening. I'm truly grateful for your attention. Please be well, enjoy your wakeful moments, and sleep well tonight. Bye for now.

23:41Thank you.