The Twelfth Mount Haemus Lecture

From solstice to equinox and back again: The influence of the midpoint on human health and the use of plants to modify such effects

by Julian Barker


Considering that all organisms on earth are united by their necessary responsiveness to the cycles generated by sun, earth and moon, I take a biological look at the constants and inconstancies of life. Health is viewed as a dynamic time series rather than a static state and one that is best achieved by acknowledging the diurnal and seasonal variations to which all living things must respond. All beings, from the earthworm to elephant, including bacteria and virus’ share the same nucleotides—DNA & RNA—which may be considered as earth’s first solar panels.

Plants, as the mediators of the sun’s energy for us all, provide not only food but the crucial templates upon which animals depend for their biochemical pathways.  Small wonder, therefore, that the health and fertility of many species of mammal depends entirely upon plants that we also use as medicine.

I review the science of circadian, tidal and annual rhythms, then move to a detailed analysis of the internal regulators of our metabolic lives and how these neurohormonal cycles are modulated by seasonal fluctuations. As a clinical plantsman, I give an overview of how the changes required from us by sun, earth and moon can be supported by the timely and judicious administration of whole plant medicines.


In these few pages I will try to outline the potential for seasonal variation in temperate regions to influence human health. My interests in biology and plant population ecology inform my approach to clinical medicine which, as a medical herbalist, draws upon the therapeutic properties of plants. There are parallels and homologies between plants and animals in their biochemistry: we all share the same environment driven by sun, water and air. I shall focus on patterns of human response to environmental fluctuations with especial reference to light intensity and day length and thence to the stretching of these adaptations at threshold seasons. Observations in Chronobiology were made in ancient and classical times and while modern interest in the subject has received intense scientific scrutiny, clinical medicine has made little use of the many useful findings.
The subject involves several disciplines. I shall try to integrate them into this paper as I do into my practice. Although the sciences of human physiology are quite abstract, their determinants are all around us in the natural world. Human health is my subject and its relations with the sun, moon, and plants. I shall turn over themes in biology and touch upon these ideas from different points of view, and hope to make for you a thread from these many strands. The form is more circular and elliptical than linear, more like the order in a game of cards than a narrative, for this is not a text–book. However, to provide some organising direction, the text divides into four sections:

1. The clocks within: adjusting from day to night
2. Heavenly time
3. The clocks within:
adjusting from winter to summer and back again
4. Adaptation:
the capacity to manage change and transition

Humans are confronted in their lives by random events yet live in a world which is shaped by predictable physical cycles. We do not know what the day has in store for us, but we know at least that it will be a day. Even when faced with a rare total eclipse of the sun, the observant human might notice that other creatures settle down for the night apparently without distress. The essential focus of this paper is on our resolution of these seemingly opposite conditions of life, a resolution that we call health.

1: The clocks within: adjusting from day to night

Health and rhythmic change

Health from a biological point of view may be defined as the fitness of an individual to match challenges from the environment. Some of these challenges will be random, or appear to be so, and therefore unpredictable. Our fitness to manage random fluctuations in our environments seems to be enhanced if we are able to conserve stable strategies for meeting those fluctuations which are predictable. The tricky manoeuvre is to make our adaptive behaviour flexible enough to deal with alteration yet regular enough to resist responding to every perturbation. In other words, our health requires that we are adequately sensitive to external events while at the same time capable of ignoring some signals. For survival, we need both healthy concern and healthy indifference; the trick is to learn when to invoke one and not the other. Health requires us to develop a buffer zone that is robust without rigidity.

An important attribute of the healthy response, then, is the ability to discriminate between isolated chance events and predictable change. One cannot judge whether an event is part of a pattern unless one has an internal pattern book against which it can be matched. Only then can the secondary assessment be made about whether the event is a risk or an opportunity.

Fluctuations in our physical external environment challenge the stability of our internal environment. These environmental fluctuations are predictable insofar as they are rhythmic. The behaviour of most creatures is determined by these fluctuations and so, in the pre-technological age in which we evolved would give us clues about the availability of food. Aperiodicity is potentially dangerous because it is unknown; periodicity can be accommodated because known. Such knowledge may lead to prey and alert us to the danger of predators.

The most important observable and predictable rhythms in nature derive from the movements of the 3 bodies: earth, sun and moon

1. Earth's spin produces a single dian period divided into the binary cycle of night and day

2. Emergence from one phase of the cycle into its opposite, produces transition zones: the narrow crepuscular phases of dusk and dawn

3. Earth's tilt produces in high latitudes the binary cycle of winter and summer

4. Emergence from each phase of this longer cycle also produces transition zones: the seasons of autumn and spring

5. The movement of tides generated by the moon influence most creatures, especially those who feed in the transition zones between land and sea

6. The annual cycle derived from earth’s orbit is a cumulative measure of ageing and success in feeding and reproduction.

The observation of such rhythms after the event serves no adaptive purpose. For organisms to use the information, they must be able reliably to predict it and to be alert before its onset. For such an operational device, there must exist some internal analogue of external periodic events. While the existence of such a timing mechanism was postulated in ancient times, it was not located until late in the twentieth century and its functions validated experimentally. The paired structures in the forebrain close to other well–known regulatory centres consist of about 8000 nerve cells each. It is unlikely that there is this one body–clock: rather it seems an indispensable regulator of all the other pacemakers and oscillators that are distributed throughout the body. These determine rhythmic vital functions such as respiration, heartbeat and temperature control, as well as the less determinate period needs of feeding and excretion. To be adaptive, all oscillators need to perceive and respond to rhythmicity in the external world, and to learn from the outcomes. To respond effectively, the clock itself must be able to adjust its rhythm to changes in the environment. These changes must be periodic: it would be of no value responding to yesterday’s weather, (especially in the British Isles). The six cycles listed above do not vary measurably within a human lifetime while many other cycles are too variable to measure. Even temperature is only secondarily a useful predictor when conjoined with the primary external cues, night and day: the regular alternation between the photophase and scotophase.


Variability is a signal characteristic of living organisms, and variance—the range of characters which any individual may exhibit and the bounds within which any individual may inhabit—provides an important measure for biologists. In this respect, living beings resemble the weather, which is forever changing but within certain bounds: winds of 400mph have never been recorded on this planet, though they are a constant manifestation on the surface of Jupiter. Weather derives in turn from climate which shows a much slower variability, thus climatic change is generally more predictable than the weather. Climatic change is likely to alter the bounds of variance of weather in a particular place over a timescale usually measured in decades and often longer than a human lifespan. Climate derives in the main from the obliquity of the earth’s axis about its orbital path around the sun, but is also profoundly influenced by latitude and other geographical effects, especially proximity to thermal reservoirs. Earth's spin and orbital velocity have changed and will continue to do so but with predictable and astronomically slow periods. There are cycles on the surface of the sun which seem to affect weather but other solar cycles and those within the earth’s magnetic core probably influence climate, but these have long periods which we are not yet able to predict. Other changes in the crust and mantle produce both slow and fast effects like tectonic drift, which cannot affect human lifetimes, and earthquakes and volcanism which do very often in all our lifetimes. Solar irradiance decreases with latitude simply because of the spherical curve of the earth but while high latitudes receive less irradiance, their seasonal climates derive more from the tilt of the earth’s axis. While this angle of obliquity varies a little, the period of this variation is 41,000 years. Together with the equinoxes which precess every 22,000 years, these variations, whatever their effects on climate, must place negligible strains upon human adaptability, to say nothing of the fluctuation in the eccentricity of earth’s orbit which clocks in at 96,000 years. None of these fluctuations could be detectable by direct human perception.

Weather and climate provide us with expected bounds of variance against which we may plan our lives. The measure of predictability is itself variable, depending upon where we live. By accommodating variability we reduce the  adaptive strain upon ourselves, that is: if we adapt we remain healthy, if not, we become ill. Accommodation involves both anticipatory behaviour and internal mechanisms which constantly sample the outside world and calibrate responses, and our responsiveness. Most of our adaptive mechanisms reduce our responsiveness to predicted change and increase our responsiveness to unexpected change. If you live in Britain, you will know that cyclonic low pressure weather tends to dominate over more stable anti–cyclonic systems. It has been shown that some individuals are very susceptible to the alternation between these two periods of weather, and that it is the relative speed and the amplitude of the oscillation at the boundaries of the two phases that causes their transient illness, manifesting as headache, nausea, visual and balance disturbances, irritability and other disorders of mood.

Health as a state of stable equilibrium in the face of variability is a biological and ecological idea to which modern medicine has yet to return.

To continue reading the paper, please download the pdf below.

The Author

The practice of herbal medicine is Julian Barker’s third (and last) profession, a career he embarked upon in the early 1970s, first in North and Central America, then came back to Britain and started all over again: training with the National Institute of Medical Herbalists and graduated from their School of Herbal Medicine. He taught Plant Biology there for 20 years during which time he wrote: The Medicinal Flora - a Field Guide to the Medicinal Plants of Britain & Northwestern Europe (Winter Press 2001). He supervised dissertations for the BSc in Herbal Medicine, and taught Botany as well as Philosophy on the MSc in herbal medicine at the University of East London, and published History, Philosophy and Medicine - Phytotherapy in Context.  As for clinical work, in 1983 he took over a retiring GP’s premises in Hove to run a multidisciplinary practice from where he has just moved to larger premises in Lewes. Since 1998, Julian has worked and studied in Paris with Dr Jean–Claude Lapraz on the application of medicinal plants according to the neuroendocrine theory of terrain.

This lecture will be published in printed form in 2016 in The Mount Haemus Lectures - Volume Two available through our bookshop.

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