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

Imagine driving through the centre of a large city at 4pm. Now imagine the same trip, but at 4am. The difference between peak-hour slog and pre-dawn jaunt exemplifies the daily variation in human behaviour - late afternoon we're up and about fighting for space on the roads, while in the early morning most of us are in bed.

This observation becomes interesting when we realise that dramatic 24-hour rhythms in activity are not simply a reaction to environmental conditions or social norms, but are driven endogenously (from within) by a self-sustaining biological clock. Humans share this clock (the "circadian system") with almost all other living things. The clock is adapted to prepare the organism for daily changes between light and dark, and is known to control a huge range of physiological and behavioural functions. Till Roenneberg's book is an engaging and informative layman's introduction to circadian science and its implications for contemporary humans.

In 24 chapters (get it?) Roenneberg uses the strategy of problem-based learning to introduce the reader to all major features of the circadian system. Each chapter commences with a brief vignette demonstrating an aspect of clock function/dysfunction. With the possible exception of one murder-mystery narrative, these vignettes work well as set-ups for information about the interaction between internal time, social time and sun time.

The first argument to prove is that ubiquitous 24-hour rhythms in physiology and behaviour are in fact driven by the clock. The notion is introduced with an anecdote about the 18th-century French astronomer Jean-Jacques d'Ortous de Mairan, who found that his mimosa plant maintained its daily rhythms in a light-proof box. In a humanising twist, we hear that de Mairan's excitement about this discovery triggered one of his frequent episodes of insomnia. Later, we hear about a family of late 20th-century laboratory hamsters with unusually fast body clocks, and how interbreeding and transplantation confirmed that their behavioural rhythms were caused by the clock.

Other chapters introduce the potential for desynchronisation between the numerous internal and external rhythms orchestrated by the clock. The human symptoms of this desynchronisation are represented by jetlagged Bostonians in Tokyo and shift-working call-centre sweethearts in Mumbai. We learn about individual difference in preferred circadian phase (chronotype, or larks versus owls), and how differences in chronotype impact performance, quality of life and even relationships. An interlude in the environment of evolutionary adaptedness ("cave-man days") is used to consider a hypothesis for why the clock shifts a couple of hours later at puberty, only returning to a more normal phase in early adulthood.

Roenneberg does an excellent job of structuring the roll-out of circadian science. Later chapters introduce more complex features, such as the timing of sleep (which involves both circadian and time-since-sleep factors). We also hear how chronotype is determined by interactions between clock period and the challenge of entrainment to sun time (larks have faster-running clocks, owls slower). Chronic sleep deprivation owing to mismatch between internal and social time ("social jet lag") is introduced as pervasive in Western life, and Roenneberg explains why this problem is exacerbated for those forced to sleep against the dictates of internal time.

This is not the first popular science book about the biological clock. Part of the appeal of the circadian system may lie in the fact that it instantiates the Romantic notion that humans are part of nature. The clock is a primordial sunrise-forecasting system, but is adapted to support, and remains exquisitely responsive to, human goal-oriented activity. By integrating quality scientific exposition with well-rounded human vignettes, Roenneberg's book shows how sophisticated human behaviours arise partly from our embodied earthly nature.