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Parallel Measurement of Circadian Clock Gene Expression and Hormone Secretion in Human Primary Cell Cultures
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Circadian clocks optimally adapt to sunlight for reliable synchronization.

Yoshihiko Hasegawa1, Masanori Arita

  • 1Department of Biophysics and Biochemistry, Graduate School of Science, The University of Tokyo, , Tokyo 113-0033, Japan.

Journal of the Royal Society, Interface
|December 20, 2013
PubMed
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Circadian clocks synchronize to daylight but need both entrainability and regularity. Optimal clock design reveals a

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circadian clockphase–response curvevariational method

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Area of Science:

  • Chronobiology
  • Mathematical Biology
  • Systems Biology

Background:

  • Circadian rhythms offer evolutionary advantages by synchronizing internal biological processes with the external light-dark cycle.
  • Maintaining a reliable circadian clock requires balancing entrainability (synchronization to external cues) with regularity (precise internal period).
  • These two properties are often in conflict, as increased sensitivity for entrainment can compromise the clock's regularity.

Purpose of the Study:

  • To investigate the conditions under which circadian clocks can achieve both high entrainability and regularity.
  • To analytically determine the optimal phase-response curve for circadian clocks.
  • To understand the relationship between clock properties and adaptation to the natural daylight cycle.

Main Methods:

  • Analytical calculation of the optimal phase-response curve using a variational method.
  • Modeling circadian clock responses to periodic stimuli.
  • Comparison of responses to actual solar radiation patterns versus simple sinusoidal stimuli.

Main Results:

  • The study identified an optimal phase-response curve that includes a 'dead zone.'
  • This dead zone represents a period where external stimuli do not alter the clock's phase.
  • A dead zone emerges specifically with time courses mimicking actual solar radiation, not with simple sine waves.

Conclusions:

  • The existence of a dead zone is a key characteristic of optimally adapted circadian clocks.
  • Circadian clocks optimally adapted to the daylight cycle possess this dead zone.
  • This finding reconciles the conflicting demands of entrainability and regularity in biological timekeeping.