Circadian Rhythms and the Human

D. S. Minors, J. M. Waterhouse

• Publisher: Butterworth-Heinemann
• Year: 2013
• Print ISBN: 9780723605928
• eBook ISBN: 9781483193427
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Circadian Rhythms and the Human covers the basic principles behind the human circadian rhythms. This book is composed of 12 chapters that discuss the detection, analysis, and definition of rhythms, specifically exogenous and endogenous rhythms. This book also demonstrates the mechanism of metabolic and gastrointestinal rhythms. The opening chapters deal with the rhythms in living organism; establishing the endogeneity of rhythms; definition of nychthemeral rhythm; methods of measuring the frequency of rhythms; exogenous effects upon the temperature rhythm; interaction between exogenous and endogenous influences; and possible origins of renal rhythmicity. The succeeding chapters consider the effect of exercise at different times of day and the concept of sleep-wakefulness rhythm. The discussion then shifts to the effects of repeated time-zone transitions and the effects of time on drug administration. The closing chapters are devoted to the assessment of work performance during shift work. The book can provide useful information to doctors, students, researchers, and the general reader.

chapter 1

Introduction and Methods of Study

Publisher Summary

A rhythm has been defined as a sequence of events that repeat themselves through time in the same order and at the same interval. The most ubiquitous feature of nature is that of rhythmicity. Rhythms are widespread in biology and are found throughout the entire evolutionary cycle, from simple unicellular organisms to complex multicellular organisms, in both the plant and animal kingdoms. In humans, rhythms oscillate once per fraction of a second, once per several seconds, and so on, through to those that oscillate once a year—the circannual rhythms. In most living organisms, however, those rhythms that predominate are the rhythms that oscillate with a frequency corresponding to that of a major environmental periodicity. Thus, many species that inhabit the shoreline exhibit rhythms that derive their frequencies from some aspect of the changing tides that result from the combined effects of the regular movement of the moon around the earth and the earth around the sun. In describing a circadian rhythm, it is often necessary to define the conditions under which the rhythm is observed. Thus, a nychthemeral rhythm refers to a bodily rhythm measured in nychthemeral conditions. This chapter reviews the detection and analysis of rhythms. It discusses exogenous and endogenous rhythms and the experiments to establish the endogeneity of rhythms. A rhythm can rarely be described as wholly endogenous or exogenous; rather rhythms usually result from an interaction of endogenous and exogenous influences though there are variations in the strengths of these components among different rhythms.

1 Rhythms in Living Organisms

Perhaps the most ubiquitous feature of nature is that of rhythmicity. Most men must be at least aware of the rhythmical nature of their environment, for in most communities social behaviour is organized to the 24-hour day and agricultural or horticultural activities require a knowledge of the alternation of the seasons. Rhythmical fluctuations occur not only at an environmental level but also at every level of nature’s organization—even at the atomic level, a property which has been exploited recently with the advent of quartz watches and atomic clocks. It is not surprising, therefore, that rhythms are widespread in the whole arena of biology and are found throughout the whole evolutionary series, from simple unicellular organisms to complex multicellular organisms, in both the plant and animal kingdoms (Aschoff, 1963; Palmer, 1976). The study of these biological rhythms has itself become a science termed ‘Chronobiology’.

That such rhythmicity occurs in man was, to a large extent, rejected in the mid-nineteenth century following the many exhaustive studies of Claude Bernard. At this time physiologists were urged to consider the body’s internal environment (milieu intérieur) as constant and resisting any change in the external environment. This theme was further taken up by Walter B. Cannon who, in his book The Wisdom of the Body, introduced a new term, homeostasis, to describe the constancy of the internal environment, although he does qualify this new word: ‘the word does not imply something set and immobile, a stagnation. It means a condition—a condition which may vary, but which is relatively constant’ (Cannon, 1939).

Today, although the basic concepts of homeostasis hold true, we envisage the internal environment to be constantly changing with, in many cases, a regular oscillatory behaviour. In certain cases (see, for example, Chapter 2) this rhythmicity results from rhythmic variations in the set-point of homeostatic mechanisms, whereas for others, rhythmicity is superimposed upon the homeostatic mechanism (for example, the secretion of certain hormones, see Chapter 7).

The frequencies of rhythms in nature cover our every division of time. Thus, in man we find rhythms which oscillate once per fraction of a second (for example, the alpha rhythm of the electroencephalogram), those which oscillate once per several seconds (for example, the respiratory rhythm) and so on, through to those which oscillate once a year, the circannual rhythms (for review, see Reinberg, 1974). In most living organisms, however, those rhythms which seem to predominate are those which oscillate with a frequency corresponding with that of a major environmental periodicity. Thus many species which inhabit the shoreline exhibit rhythms which derive their frequencies from some aspect of the changing tides that result from the combined effects of the regular movement of the moon around the earth and the earth around the sun. Similarly, the annual alternation of summer and winter in temperate latitudes, which results from the movement of the earth around the sun, gives rise to the annual reproductive behaviour of plants and hence often also of the animals dependent upon them for their food. For most living organisms, however, and most certainly for man, the most evident environmental change is that which results from the regular spin of the earth about its central axis, namely the alternation between day and night—the solar day—which oscillates with a frequency of once per 24 hours. Since our habits of sleep, rest and activity, work and leisure, eating and drinking, largely follow a routine which is governed by the alternation of day and night and the social organization which results from this, it is hardly surprising that many human processes, physiological, psychological and biochemical, oscillate with a frequency similar to that of the solar day. It is these rhythms that have become known as circadian, from the latin circa (about) and dies (a day) (Halberg, 1959).

2 Definition of Terms

As with other fields of study, those who study circadian rhythms have developed specialist terms to facilitate concise communication and to avoid unwieldy descriptions. A glossary of terms used in this text is to be found at the end of the book (pp. 320–21), but for a fuller description of the entire vocabulary of terms used one should consult the Glossary of Chronobiology (Halberg and Katinas, 1973; Halberg et al., 1977). The following section, however, will briefly describe those terms which will be used frequently; other terms will be described when they arise in the text.

Before describing variables which exhibit circadian rhythmicity and the mechanisms which control these rhythms, it is of course necessary first to understand what is meant by a rhythm.

A rhythm (synonyms are oscillation, cycle, periodicity) has been defined by some (for example, Palmer, 1976) as a sequence of events that repeat themselves through time in the same order and at the same interval. Though this definition is true for a stationary time series in the long term, it should not be interpreted as meaning that a rhythm cannot change its characteristics; rather, a biological rhythm may, for example, change its cycle length (for example in the transition from the entrained state to a free-running state, see s. 6.2).

Some simple waveforms which may represent a rhythm are shown in Fig. 1.1. Such rhythms may be quantified as shown in Fig. 1.1 by:

Fig. 1.1 Different forms of a rhythm. A, sinusoidal; B, symmetrical but non-sinusoidal; C, asymmetrical, a = acrophase; p = peak; A = amplitude. The vertical lines divide the traces up into two cycles. (From Conroy and Mills, 1970, Fig. 0.1.)

1. The period of the rhythm. This is the reciprocal of the frequency and is the time to complete one cycle. Halberg (1959) has defined a rhythm as being circadian if its period lies in the range 20–28 hours. Rhythms with a period less than this are then termed ultradian and those with a longer period, infradian.

2. The mean or level of the rhythm. This is the average value of a continuous variable over a single cycle. When the rhythm is described by the fitting of a cosine curve (see s. 4) the level of the best-fitting cosine curve is known as the mesor. Only when the data are measured equidistantly and over an integral number of cycles will the mesor equal the arithmetic mean.

3. The amplitude of the rhythm. In its strictest mathematical usage the amplitude refers to the magnitude of the variable between its mean value and the trough or peak. Such mathematical usage, however, is limited to rhythms which oscillate symmetrically about the mean level and would be inappropriate, for example, to the waveform shown in Fig. 1.1B. The term ‘amplitude’ has consequently been used by some to designate the range of oscillation from peak to trough. To avoid ambiguity, therefore, it is suggested that the term ‘amplitude’ should be used when the rhythm is represented by a symmetrical mathematical model and otherwise ‘range of oscillation’ be used.

4. The phase of the rhythm. This word has several meanings. In its strictest definition, the phase refers to the instantaneous state of the rhythm within a cycle, represented by the value of the variable—for example, the maximum or minimum. The phase of the rhythm thus tells us of the position of the rhythm in time. (In the special case where the data are represented by a fitted cosine curve the phase of the rhythm may be defined by the acrophase which is the time of maximum of the cosine function.) A phase-shift implies that whilst the rhythm retains a similar shape, it is displaced along the time axis. A phase-shift can be further described as a phase-advance or phase-delay to describe the direction of the displacement along the time axis. Thus a phase-delay means that every point in the rhythm occurs somewhat later, whilst a phase-advance means that every point occurs earlier. The term ‘phase’ is also used to describe the temporal relationship between two rhythms. When unqualified, for example, ‘the rhythm of deep body temperature was in phase with the rhythm of activity’, this means that similar aspects of the two rhythms (for example the times of maxima) occurred simultaneously. (When phase is used in this sense, the opposite, when the maximum of the one rhythm would occur simultaneously with the minimum of the other, is called antiphase.)

In describing a circadian rhythm it is often necessary to define the conditions under which the rhythm was observed. Since often rhythms are measured i...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Acknowledgements
• Foreword
• Chapter 1: Introduction and Methods of Study
• Chapter 2: The Circadian Rhythm of Deep Body Temperature
• Chapter 3: Cardiovascular, Respiratory, Metabolic and Gastrointestinal Rhythms
• Chapter 4: The Kidney and Hormones Affecting it
• Chapter 5: Sleep and Wakefulness
• Chapter 6: Rhythms in Mental Performance
• Chapter 7: The Endocrine System
• Chapter 8: Rhythms in the Infant and the Aged
• Chapter 9: Time-Zone Transitions
• Chapter 10: Shift Work
• Chapter 11: Clinical Implications
• Chapter 12: Circadian Rhythm Mechanisms
• Statistical Analysis of Rhythms
• glossary
• Index