Chapter

Circadian Clocks

Volume 217 of the series Handbook of Experimental Pharmacology pp 311-331

Date:

Light and the Human Circadian Clock

  • Till RoennebergAffiliated withInstitute for Medical Psychology, Centre for Chronobiology, Medical Faculty, Ludwig-Maximilians-University Email author 
  • , Thomas KantermannAffiliated withChronobiology - Centre for Behaviour and Neurosciences, University of Groningen
  • , Myriam JudaAffiliated withDepartment of Psychology, University of British Columbia
  • , Céline VetterAffiliated withInstitute for Medical Psychology, Centre for Chronobiology, Medical Faculty, Ludwig-Maximilians-University
  • , Karla V. AllebrandtAffiliated withInstitute for Medical Psychology, Centre for Chronobiology, Medical Faculty, Ludwig-Maximilians-University

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Abstract

The circadian clock can only reliably fulfil its function if it is stably entrained. Most clocks use the light–dark cycle as environmental signal (zeitgeber) for this active synchronisation. How we think about clock function and entrainment has been strongly influenced by the early concepts of the field’s pioneers, and the astonishing finding that circadian rhythms continue a self-sustained oscillation in constant conditions has become central to our understanding of entrainment.

Here, we argue that we have to rethink these initial circadian dogmas to fully understand the circadian programme and how it entrains. Light is also the prominent zeitgeber for the human clock, as has been shown experimentally in the laboratory and in large-scale epidemiological studies in real life, and we hypothesise that social zeitgebers act through light entrainment via behavioural feedback loops (zeitnehmer). We show that human entrainment can be investigated in detail outside of the laboratory, by using the many ‘experimental’ conditions provided by the real world, such as daylight savings time, the ‘forced synchrony’ imposed by the introduction of time zones, or the fact that humans increasingly create their own light environment. The conditions of human entrainment have changed drastically over the past 100 years and have led to an increasing discrepancy between biological and social time (social jetlag). The increasing evidence that social jetlag has detrimental consequences for health suggests that shift-work is only an extreme form of circadian misalignment, and that the majority of the population in the industrialised world suffers from a similarly ‘forced synchrony’.

Keywords

Chronotype Entrainment Sleep Zeitgeber Zeitnehmer Free-running period Clock evolution