Early birds and night owls are born, not made. Sleep patterns may be the most obvious manifestation of the highly individualized biological clocks we inherit, but these clocks also regulate bodily functions from digestion to hormone levels to cognition. Living at odds with our internal timepieces, Till Roenneberg shows, can make us chronically sleep deprived and more likely to smoke, gain weight, feel depressed, fall ill, and fail geometry. By understanding and respecting our internal time, we can live better.
Internal Time combines storytelling with accessible science tutorials to explain how our internal clocks work-for example, why morning classes are so unpopular and why "lazy" adolescents are wise to avoid them. We learn why the constant twilight of our largely indoor lives makes us dependent on alarm clocks and tired, and why social demands and work schedules lead to a social jet lag that compromises our daily functioning.
Many of the factors that make us early or late "chronotypes" are beyond our control, but that doesn't make us powerless. Roenneberg recommends that the best way to sync our internal time with our external environment and feel better is to get more sunlight. Such simple steps as cycling to work and eating breakfast outside may be the tickets to a good night's sleep, better overall health, and less grouchiness in the morning.
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About the Author
Till Roenneberg is Professor at the Institute of Medical Psychology at the Ludwig-Maximilians University, Munich.
Read an Excerpt
Chapter 15: When will my organs arrive?
In our modern world of traveling, millions of people have suffered from jet lag at least once in their life: when visiting friends or when taking their vacations in distant countries. People whose jobs take them to many different parts of the world find themselves in this lousy state with sad regularity. But what are the symptoms that characterize jetlag a bit more accurately than just feeling lousy? The most conspicuous symptom is tiredness. However, that state is not necessarily specific for jet lag. If someone traveled from Helsinki to Cape Town, he or she might also feel severely tired without having ever left the time zone, merely due to the length of exhausting trip (not to mention that this traveler would also travel from winter to summer or from autumn to spring or vice versa). The major difference between traveling long distances within time zones, opposed to across time zones, is that the passenger from Helsinki would have no problems in sleeping at the right time of night in Cape Town and would thus recover quite readily from the exhausting voyage. But merely traveling long distances across time zones also does not necessarily throw the traveler into the state of jet lag. In the old days, before airplanes carried the travelers across the Atlantic, they may have suffered from seasickness but certainly not from jet lag. As the word says, it takes a jet to elicit this state because the main cause for this syndrome is the speed at which we travel from one time zone to another, from one time of dawn to another, from one time of dusk to another, and from one different social timing to another. Our body clocks can cope with the slow changes of dawn and dusk we would experience when traveling by ship. If we took a boat from Europe to America, it would actually be a bit like living on Mars: every evening the sun would set and every morning it would rise a bit later, making the days longer than 24 hours. Of course, we would much more easily synchronize to these longer days if we spent some time on deck—especially in the evenings—thereby exposing ourselves to bright light during the time of our body when light expands the internal day. If we took a boat in the other direction, from America to Europe, we would live on yet another planet with days shorter than 24 hours by traveling against the rotation of our globe. If the boat didn’t cruise too fast across the Atlantic, these shorter days would still be within the range of entrainment of our body clock so that our internal time would arrive in synchrony with the external time. According to the principles of entrainment, we would become slightly earlier chronotypes when traveling west and slightly later chronotypes when traveling east.
However, when it takes us less than a day to travel across half of the globe, our body clock is left behind. The shortest flight from Boston to Tokyo takes 15 hours and 25 minutes. If Oscar and Jerry had left Boston’s Logan airport at 8 A.M., it would already be 9 P.M. in Tokyo and they would arrive at Narita airport half an hour after noon local time (it would actually be a whole date-day later since they would have crossed the date line). Their internal time would, however, be set to thirty minutes before midnight—approximately half a day out of synch.
Another symptom of jet lag is nighttime insomnia despite utter exhaustion. Based on the calculations above, this is not surprising. Unless we have traveled to our holiday destination, we are expected to be active when our body clock is on its way to bed and have to try and catch up on sleep when our internal alarm clock “announces” the time to get up. As a rule of thumb, it takes the body clock approximately one day per travelled time zone to adjust to the new cycle of light and darkness, so that Bostonians traveling to Japan would need approximately 12 days until they functioned normally again. The difficulty of not being able to sleep adds to the exhaustion of the trip itself. Since this state may continue for many days after arrival at the destination, the traveler cannot compensate for the exhaustion experienced during the long flight.
Table of Contents
1 Worlds Apart 8
2 Of Early Birds and Long Sleepers 16
3 Counting Sheep 24
4 A Curious Astronomer 31
5 The Lost Days 36
6 The Periodic Shift Worker 47
7 The Fast Hamster 60
8 Dawn at the Gym 66
9 The Elusive Transcript 75
10 Temporal Ecology 81
11 Wait until Dark 90
12 The End of Adolescence 96
13 What a Waste of Time! 106
14 Days on Other Planets 114
15 When Will My Organs Arrive? 129
16 The Scissors of Sleep 139
17 Early Socialists, Late Capitalists 152
18 Constant Twilight 163
19 From Frankfurt to Morocco and Back 173
20 Light at Night 184
21 Partnership Timing 193
22 A Clock for Ail Seasons 202
23 Professional Selection 214
24 The Nocturnal Bottleneck 223
What People are Saying About This
Internal Time is an accessible, up-to-date overview of a subject that is important to all of us. With its remarkable depth and breadth of coverage, this book should be of interest to a wide and diverse audience.
Martin Zatz, Editor, Journal of Biological Rhythms
This is a wonderful book from a gifted scientist, thinker and writer that provides the reader with the rare opportunity to discover something new about themselves and the world in which they live.
Russell G. Foster, University of Oxford