Circadian Rhythms and Hormonal Cycles

Many hormones are not secreted at a constant rate but rise and fall on a roughly 24-hour schedule set by an internal clock. A central pacemaker in the hypothalamus synchronises these rhythms to the day-night cycle, so that hormones such as cortisol, growth hormone, and melatonin peak at characteristic times.

Definition

Hormonal circadian rhythms are approximately 24-hour cycles of hormone secretion generated by an internal molecular clock and synchronised to the environment by the hypothalamic suprachiasmatic nucleus.

Scope

The topic covers the molecular and anatomical basis of the mammalian circadian clock, the role of the suprachiasmatic nucleus as the master pacemaker, how the clock imposes daily rhythms on hormone secretion, and the distinction between circadian, pulsatile, and longer hormonal cycles. It is a physiology topic, not clinical guidance.

Core questions

• What generates a circadian rhythm at the molecular level?
• How does the suprachiasmatic nucleus act as the master clock?
• Which hormones show prominent daily rhythms and when do they peak?
• How do circadian rhythms relate to pulsatile and longer hormonal cycles?

Key concepts

• Suprachiasmatic nucleus (master clock)
• Clock genes and the transcription-translation feedback loop
• Entrainment to light
• Cortisol diurnal rhythm
• Melatonin and the sleep-wake cycle
• Nocturnal growth hormone secretion
• Pulsatile versus circadian secretion
• Ultradian and infradian cycles

Key theories

Transcription-translation feedback loop

The cellular circadian clock is built from a set of clock genes whose protein products inhibit their own transcription; the delay in this negative-feedback loop produces a self-sustaining oscillation with a period of about 24 hours.

Central pacemaker synchronising peripheral rhythms

The suprachiasmatic nucleus, entrained to light by the retina, coordinates clocks throughout the body and imposes daily timing on neuroendocrine output, aligning hormone rhythms with the day-night cycle.

Mechanisms

Within individual cells, a core set of clock genes forms a transcription-translation feedback loop: activating proteins drive expression of repressor genes whose products then inhibit the activators, and the built-in delay yields a self-sustaining oscillation of about 24 hours. The suprachiasmatic nucleus of the hypothalamus contains a network of such clock cells, is reset each day by light signals from the retina, and acts as the master pacemaker that synchronises peripheral clocks and neuroendocrine output. Through neural and hormonal routes the suprachiasmatic nucleus imposes daily timing on the hypothalamic-pituitary axes, so cortisol peaks near waking, growth hormone is released mainly in early deep sleep, and melatonin rises at night. These circadian rhythms are superimposed on faster pulsatile secretion and on longer cycles such as the menstrual cycle.

Clinical relevance

Because hormone levels vary predictably across the day, the time at which a hormone is measured is part of interpreting its value, and disruption of circadian timing is recognised as physiologically consequential. This entry describes the timing of normal secretion and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The molecular clock model and the pacemaker role of the suprachiasmatic nucleus are established in widely cited reviews; the discovery of the clock-gene feedback mechanism was recognised with the 2017 Nobel Prize in Physiology or Medicine. The coupling of the clock to endocrine output, particularly the HPA axis, is summarised in dedicated reviews.

History

Daily rhythms in physiology were long observed, but their endogenous, genetically based nature was demonstrated in the late twentieth century. Identification of the suprachiasmatic nucleus as the mammalian master clock and the subsequent cloning of clock genes revealed the transcription-translation feedback loop, providing a molecular mechanism for the daily timing of hormone secretion.

Key figures

• Joseph S. Takahashi
• Steven M. Reppert
• Michael W. Young
• George P. Chrousos

Seminal works

• reppert-weaver-2002
• takahashi-2016

Frequently asked questions

What is the body's master clock?

The suprachiasmatic nucleus, a small region of the hypothalamus. It generates a self-sustaining roughly 24-hour rhythm, is reset daily by light, and synchronises clocks and hormone rhythms throughout the body.

Why does cortisol vary across the day?

The circadian clock drives the HPA axis so that cortisol secretion rises to a peak around the time of waking and falls to a trough at night, producing its characteristic daily rhythm.