130 Time’s Books

Till Roenneberg Internal time: , Social , and Why You’re So Tired. Cambridge, MA: Harvard University Press, 2012. 288 pp.

Till Roenneberg’s book Internal Time deals with the body’s internal clock and its interaction with external “” like sun time or social time. This book was originally written in German and published by DuMont Buchverlag, Cologne, , in 2010. Roenneberg divides his book into 24 chapters, each of which begins with a case story that is fictitious but borrows much from both real and sometimes personal experiences and from scientific data. The author uses this stylistic device, as he says, to raise the reader’s curiosity and to trig- ger her or his reason. The second part of each chapter describes the facts that underlie the case stories in detail. They aim at answering the possible ques- tions that might come from reading the case stories. Roenneberg’s hypothesis about internal time is that it controls most (if not all) of our body functions, ranging from genes being activated at certain times, to changes in body temperature and hormonal composition, up to cognitive functions like the ability to do math. The 24 chapters all are dedicated to the phenomenon of internal time and its connection to behavior, society, or . I think that my outlining each chapter in a short and comprehensible manner may satisfy the needs of intrigued people and potential purchasers more than a global compendium because each chapter reflects a unique perspective on the issue of internal time. In the first chapter, Roenneberg introduces the construct of chronotypes. This construct classifies human beings depending on their sleep and wake-up habits. In the extremes, people rise early in the morning, making them chrono- type A (or larks, as Roenneberg calls them); on the other side of the continuum, people go to sleep late at night (and correspondingly get up late in the morn- ing) and are labeled B (or owls, as Roenneberg says). Roenneberg makes a distinction between sleep timing and sleep duration because both are in principle independent from one another. Chapter 2 is about early-morning moral judgments that are quite often pres- ent in Western societies: that productivity is predominantly restricted to those hours of the day that are lit up by the sun. The opinion that early risers are good people and that late risers are lazy has its roots in rural societies, but it becomes questionable in modern 24/7 societies. Chapter 3 is about sleep. Sleep is regulated by at least two independent components—by our being tired and by the time of the day. Roenneberg introduces a model proposed by sleep researchers Alex Borbely and Serge

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Daan.1 According to this model, two different kinds of rhythms are produced by different oscillators, one behaving like an hourglass, and the other like a pendulum. While an hourglass has to be actively turned once its top chamber is empty, a pendulum swings by itself. In Borbely’s and Daan’s model, one of the chambers of the hourglass represents sleep pressure. During wakefulness, this chamber is at the bottom and is continually “filled.” When the level of sleep pressure reaches a critical threshold, we fall asleep. The hourglass is turned around, and the chamber empties until it reaches another (lower) threshold that wakes us up. The hourglass is turned around again, and the cycle restarts. This process produces a result that, when depicted graphically on a time by sleep-pressure matrix, resembles a saw-tooth pattern. In chapter 4, Roenneberg mentions the first report of a scientist (the French astronomer de Mairan in 1729) in which the independence of daily rhythms from the changes of night and day, or dark and light, were described. Chapter 5 continues with this observation and describes in detail the famous experi- ments conducted by German scientist Juergen Aschoff and co-workers who let their participants stay for a few weeks in complete isolation in a bunker lacking social and physical clocks. Aschoff and his colleagues discovered the internal desynchronization of different body rhythms in the late 1960s. They found a clear separation between the rhythms in behavior and physiology. For example, some of the subjects’ individual days were twice as long as our nor- mal 24-hour days, but their period of core body temperature rhythm and other physiological rhythms stayed close to 24 hours. Chapters 6 and 7 deal with two important aspects of the internal clock: photoreceptors as a necessary interface between internal and external tempo- ral rhythms, and the physical location of the internal clock. As has frequently been shown, light is the main signal that resets and synchronizes the body clocks of plants and animals. As scientists found that animals’ timing behavior (that is, getting synchronized to light-dark cycles) was affected although all rods and cones were removed from the retina, the search was open to find a hitherto unknown receptor in the eye—which eventually was found. This newly discovered photoreceptor is called melanopsin. In the 1970s, the ques- tion about where to find the entire clock was pursued extensively. In mam- mals, the clock was found in a small group of neurons above the optic chiasm called the suprachiasmatic nucleus. If this nucleus is removed from the brain, timing activity is immediately lost.

1 S. Daan, D.G. Beersma, and A.A. Borbely, “Timing of human sleep: Recovery process gated by a circadian pacemaker,” American Journal of Physiology 246 (1984): 161-183.

KronoScope 15 (2015) 119-137