Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired

by Roenneberg, Till

  Mercury: 1392.94

  Venus: 5771.25

  Earth: 23.93

  Mars: 24.62

  Jupiter: 9.92

  Saturn: 10.23

  Uranus: 17.73

  Neptune: 18.20

  Pluto: 151.50

  Professor Rasmunson indicated that the hugely different day lengths on the other planets created a problem for the synchronization of the human clock. She proposed to determine the chronotype of every human on Earth, which could easily be done by loading a drop of blood onto a device called the “ChronoChip.” With that knowledge one could assemble groups of similar chronotypes, and then send each of these off to a different planet. She then went into a long explanation of the formal basis of synchronizing circadian clocks—the principles of entrainment, as she called them.

  Professor Rasmunson was interrupted by one of the other scientists. “What happens if partners or family members have very different chronotypes?”

  “That shouldn’t be a very frequent scenario,” she responded. “As you know, human courtship behavior over the past 200 years has favored partnerships among similar chronotypes, thereby producing rather homogeneous families, similar to the historical family traits of social rank, ethnicity, or religion. If it were to happen occasionally, however, it might create a real problem.” She then added, “We can do a fair bit with light intensities and wearing sunglasses to adjust individual phase.” After a brief moment of contemplation, the professor added with a little chuckle, “On the other hand, some people might be quite pleased to have an excuse for a separation.” She had recently been cheated on by her husband and was in the middle of a divorce, and her remark got a couple of laughs out of the audience.

  The majority of people would surely be settled on Mars, which already had the biggest extraterrestrial colony in the Solar System. “According to the old proverb, every one of them will be healthier, wealthier, and wiser,” quipped Svenja—but since only chronobiologists could easily appreciate the joke, nobody laughed. She quickly moved to the subject of special treatment for extreme chronotypes, like those who were first found in Utah. Individuals of this chronotype could potentially be sent to Uranus and Neptune, or maybe even to Jupiter and Saturn.

  Several years later, WASPS decided to send everyone to the large Mars settlement and discarded all plans for settling on other planets. In their initial advertising campaign, WASPS even picked up the joke Rasmunson had made that day on Mont Blanc:

  Off to the stars,

  settle on Mars,

  see good old Earth rise,

  be healthy, wealthy, and wise!

  (Discounts and prime housing opportunities for early risers–fast deciders.)

  This may be the most difficult of the twenty-four chapters for some readers to digest. But please bear with me and take your time. It will be worth your while because you will only really understand how body clocks synchronize to the twenty-four-hour day on Earth if you are familiar with the formalisms underlying synchronization, called principles of entrainment. Once you have understood these principles, you will easily understand why people with different clocks have to be different chronotypes, why your chronotype depends on whether you are an office worker or a farmer, and why Svenja wants to send extreme early chronotypes to Uranus or Neptune. You may wonder why I constructed a story around life on other planets when I want to explain how entrainment works on Earth. The reason is simple: you start to understand everyday life if you step out of it.

  Few people know that the rotation of our planet has slowed down in its unimaginably long life history. This process has not stopped, so days on Earth will be close to twenty-five hours in a very, very distant future. Thus biological clocks can obviously adapt to different day lengths. This long-term adaptation is, however, different from sending people to other planets where they would arrive within months or years. The evolutionary adaptation generates different versions of clocks with altered or even new genetic components whereas the synchronization to days on other planets must be achieved via the principles of entrainment.

  Successful entrainment ensures that the body clock produces internal days that have on average the same length as the external days of the cyclic environment. You already know that the clocks of different individuals run differently (by inheritance) and may, therefore, produce days that are longer or shorter than the rotation of our planet (as if these people were made for a life on other planets). The principles of entrainment have to accommodate the following conditions in our extraplanetary scenario:

  1.If individuals’ clocks, producing internal days that are shorter than twenty-four hours, have to entrain to the rotation of Earth, then their internal days have to be lengthened.

  2.If individuals’ clocks, producing internal days that are exactly twenty-four hours, have to entrain to the rotation of Mars, their internal days also have to be lengthened.

  3.If individuals’ clocks, producing internal days that are longer than twenty-four hours, have to entrain to the rotation of Earth, their internal days have to be shortened (the most common scenario for humans).

  4.If individuals’ clocks have to entrain to the rotation of Neptune, then their internal days would really have to be shortened, even if they were producing internal days that were already shorter than twenty-four hours.

  The idea of entraining the clocks of humans to the rotation of other planets is not far-fetched: Claude Gronfier simulated light–dark cycles longer than twenty-four hours in the laboratory to find out whether the human clock could entrain to days on Mars.1 Humans can easily entrain to a twenty-four-hour day in the laboratory, even if the intensity of the light–dark cycle is relatively dim. Gronfier found that he had to increase this intensity to successfully entrain his subjects to the longer, Mars-like days.

  But before I come to the question of how we possibly could entrain to days on other planets, I want to address a more general question: how does a circadian clock entrain to a zeitgeber, or external cue?2 Technically, anything that produces a rhythm is a kind of oscillator (like a swing or a pendulum). But how can the period of an oscillator be adjusted to the period of another rhythm? Let’s presume that the body clock is a swing that is being pushed by a zeitgeber (for example, the light–dark cycle). To synchronize the body clock, the zeitgeber has to systematically interfere with the momentum of the swing. (Admittedly, a swing is a strange representation of the body clock and the pusher an even stranger impersonation of a zeitgeber.)

  At 6 A.M. the swing starts to move from left to right. It passes its lowest point at noon, reaches the other end by 6 P.M., and passes the low point again at midnight on its way back. In short, it swings from the left to the right during the day and back from the right to the left during the night. Whenever the zeitgeber pushes, the swing’s reaction will depend on when during the swing this happens. If the swing is pushed while it is moving away from the pusher, it will speed up; if it is pushed while moving toward the pusher, it will slow down or even stop.

  The motion of the swing represents the body’s internal clock, and the pusher is the zeitgeber, keeping the swing in a precise twenty-four-hour rhythm. From T. Roenneberg, S. Daan, and M. Merrow (2003). The art of entrainment. Journal of Biological Rhythms 18(3):183–194.

  If a body clock could think and talk, it might respond thus to the following situations:

  If the clock thinks that it is past midnight (internal time) but suddenly sees light, it might say, “Oh, gosh, it’s already approaching dawn. I really have to hurry up!”

  If the clock thinks it is already past sunset but then sees light, it might say, “Golly, it’s still light out there, I really have to slow down and get back on track!”

  If the clock is notified by the eyes that is broad daylight and it thinks it is midday, it might lose its patience and respond, “I know it’s day! Please bother me only if there is something to report that I don’t already know!”

  It is quite remarkable how the body clock responds to light. The experime
nters who investigated how the clock responds to light used single light pulses, each of them given at a different internal time in otherwise constant darkness (or dim light). For each of these experiments they measured how much the clock changed the course of its internal time in response to the light stimulus. These experiments have taught us a lot about how the circadian oscillator responds to light, but their experimental conditions are highly artificial. In nature, circadian clocks are not completely dark-adapted over many days and are certainly not synchronized by a daily, single short pulse of light. We have, therefore, recently extended the theory of entrainment, originally constructed with single light pulses, so that it can explain entrainment in the “noisy” light environments of the real world. This new theory is based on a “response characteristic” that shows how sensitive the clock is toward light at different times during its internal day and indicates whether light shortens (compresses) or lengthens (expands) the internal day.

  Entrainment is nothing more than making the internal day fit the external day—either by compression or by expansion. The response characteristic shows that light around internal dawn compresses the internal day (the higher the curve, the greater the compression); it has little or no effect around internal midday (the curve runs parallel to the dotted zero-line); and light expands the length of the internal day around internal dusk (the lower the curve, the greater the expansion). These specific responses allow biological clocks to synchronize with their cyclic environment. To correct for errors that may arise from the internal day being too short or too long, the body clock exposes different parts of its internal day to light and “hides” the remaining parts in the dark.

  How do the rules of this hide-and-seek game explain the numerous cryptic aspects of entrainment on other planets? What is the relationship between chronotype and the day length, Svenja Rasmunson’s main concern? The clock’s free-running period (its internal day length) is one of the reasons why we are different chronotypes. When early and late types live in temporal isolation, the clocks of the former run faster than those of the latter, producing shorter or longer internal days. If an internal day is shorter than the external day, it has to be expanded. If it is longer, it has to be compressed. What could be easier?

  Light entrains the internal day to the external day. Light around internal dawn compresses the internal day, has little or no effect around internal midday, and expands the length of the internal day around internal dusk.

  Let’s start with the most simple of all entrainment cases, an individual’s clock that produces on average internal days that are exactly twenty-four hours long. When it has to entrain to the twenty-four-hour light–dark cycle of our planet, nothing has to be changed; the clock’s internal days should be neither compressed nor expanded. On very short (winter) days, this could be achieved by “hiding” all light-responsive portions of the cycle in the long nights. However, if days are longer (as in spring or summer), the sensitive portions of the response characteristic will be exposed to light. In this case, the only way that a clock which produces exactly twenty-four-hour days can entrain is by exposing equal amounts of the compression and expansion portions to the light, so that the two opposite effects cancel each other out. Note that these are the only circumstances in which internal and external clock time are identical (the two time scales are indicated at the bottom and the top, respectively). Internal midnight matches the middle of the dark period and internal midday occurs when the sun reaches its highest point.

  An internal clock that produces exactly twenty-four-hour days entrains by exposing equal amounts of the compression and expansion portions to the light, so that the two opposite effects cancel each other out.

  Yet humans whose clocks produce exactly twenty-four-hour days are extremely rare—most human clocks produce longer days, which therefore have to be compressed. To achieve this, the clock simply exposes more of its compression portion to the light and “hides” more of its expansion portion in the dark. As a consequence, internal time will end up being a bit later than external time. The longer a clock’s internal day, the later it will “move” in relation to external time. This is why people with slow clocks have to be late chronotypes; otherwise their clocks would not match the day length on our planet.

  An internal clock with a longer day has to move to a later external time (see white arrow), thereby exposing more of the compression portion to the light and hiding more of the expansion portion in the dark. This results in a late chronotype.

  The opposite is true for clocks that produce internal days shorter than twenty-four hours. They have to move to an earlier external time, thereby exposing more of the expansion portion to the light and hiding more of the compression portion in the dark. Faster clocks produce earlier chronotypes, so that their internal noon is now earlier than external midday.

  An internal clock with a shorter day has to move to an earlier external time (see white arrow), thereby exposing more of the expansion portion to the light and hiding more of the compression portion in the dark. This results in an early chronotype.

  These two theoretical examples refer to the entrainment of individuals whose clocks produce different internal days but who all live on Earth. Since it matters only for entrainment that a clock’s day has to be expanded or compressed, it is but a small step to understand entrainment on other planets.3 If WASPS sends people to a planet that rotates faster than Earth, their internal days would have to be compressed (by moving internal time to a later external time), thereby creating later chronotypes. If WASPS sent the same people to a planet that rotates slower than Earth, their internal day would have to be expanded (by moving internal time to an earlier external time), thereby creating earlier chronotypes. Since the days on Mars are longer than the days on Earth, all of us would become earlier chronotypes, as Svenja’s joke and the WASPS advertisement suggest.

  But can our body clock adjust to the day length of any old planet? Surely not: how could we fit our activity times and our sleep needs into days that are approximately sixty times longer than ours (for example, on Mercury)? We wouldn’t be able to function without sleep for forty days, and then we wouldn’t be able to stay asleep for the following twenty days. So what are the limits within which our body clock can adapt to the days on other planets? The hide-and-seek game of entrainment makes the answer easy. The shortest day length we can entrain to is reached when the entire compression portion is exposed to light (and its entire expansion portion is hidden in the dark). And the longest day we can entrain to is reached when the entire expansion portion is exposed to light (and its entire compression portion is hidden in the dark). Let’s assume that the body clock could be compressed and expanded maximally by two hours and that its internal day was exactly twenty-four hours long. Theoretically, this clock could entrain only to days longer than twenty-two hours and shorter than twenty-six hours.4 The maximum compression and expansion capacities depend on zeitgeber strength: the brighter the day and the darker the night, the wider the range of entrainment. Remember, Claude Gronfier had to use brighter light in his laboratory experiments to entrain subjects to a Mars-like day than was necessary to entrain them to a twenty-four-hour day.

  The limits of entrainment: the shortest day we can entrain to is reached when the entire compression portion is exposed to light (above); the longest when the entire expansion portion is exposed to light (below).

  According to these principles of entrainment, we have to presume that the internal days of extreme early chronotypes are very short.5 That is why they are probably the only humans who might be able to entrain to the natural days on Uranus and on Neptune (with day lengths of 17.73 and 18.2 hours, respectively). In this highly theoretical scenario, these extreme early birds on Earth would become extremely late chronotypes on Uranus and Neptune, much later than the latest teenagers on our planet. Even so, it is unlikely that the internal days of these extreme early chronotypes would be as short as eighteen hours, making their outing to Neptune nothing more than a c
hronobiological fantasy. And yet, some mutations do have drastic effects on the length of internal days.

  I now have covered all of the entrainment issues hidden in my tale of life on other planets except for one: why did Svenja Rasmunson suggest at the end of her presentation that extreme early types could even be sent to Jupiter and Saturn, which are characterized by day lengths of 9.92 and 10.23 hours, respectively? Surely even extreme early types cannot get up every nine to ten hours, be awake for six hours, and sleep for only three hours. This regime would resemble what Sergeant Stein had to live through—without much success, as we know. But interestingly, this wouldn’t happen. The body clock would adapt to these extremely fast cycles by what chronobiologists call frequency demultiplication. You know frequency demultiplication well from everyday life situations. When people clap to music in concerts, they often clap on every other beat. If you walk swiftly with a stick or an umbrella, you poke it onto the ground every other step. On Jupiter or Saturn, the clock of extreme early types would simply skip every other day as if the second dark period was the result of a regular thunderstorm in the afternoon. Inhabitants would probably adopt a Mediterranean lifestyle on Jupiter or Saturn: they would sleep-deprive themselves during one of the (short) nights and have a nice long siesta during the other. Yet only extreme larks could live such a life on Jupiter or Saturn because “normal” clocks would fail to entrain to the eighteen-to twenty-hour double days on these planets.