In Seasons of Life, Leon Kreitzman and Russell Foster explain why the seasons occur, the impact of seasonal change and how organisms have evolved to anticipate these changes. In this guest post from Kreitzman, SciScoop readers get an exclusive insight into the concepts behind the book.

The natural world is full of rhythms. How do birds know when to return to their nesting grounds? What effect do the seasons have on our wellbeing, and how does the season in which we are born affect our subsequent life chances? How did humans get the idea that there were seasons 50,000 years ago?

In our modern society, even with all the contrivances we use to dominate nature and hide from seasonal change, a persistent seasonal variation in births is an almost universal characteristic of human populations, whether urban or rural, tropical, temperate, or boreal.

In the past two hundred or so years since the Industrial Revolution, the amplitude of the variation has decreased in most countries as a result of secular trends. No doubt social, cultural, economic and religious factors account for much of the change in variation, but there is considerable evidence that even in our hyper-modern world biological factors still can affect the timing of human fertility.

Even though the human male and female are reproductively competent throughout the year, reproductive hormones in both men and women show a marked seasonal variation. Despite our being opportunistic breeders, there seems to be a ‘best’ time for human babies to be born. And surprisingly, our birth timing may affect not only affect infant survival but also the diseases we nay get in later life and even how long we live!

We find it hard to imagine that the regular, predictable seasonal variation in the length of the day (photoperiod) or the night (scotoperiod), which is the most reliable indicator of seasonal timing, may still profoundly affect our biological make-up. We readily accept that the breeding rhythms of sheep, the flowering of plants, the migration timings of caribou, birds and Monarch butterflies and the hibernation of squirrels are synchronised to the seasonal photoperiod in their natural surroundings. But the idea that modern day humans may still be influenced, even partially, by a similar process, with the consequences that our life-cycle health and well-being are dependent on our birth timing is intriguing.

The ability to anticipate regular daily and seasonal changes in the environment is of huge survival value to plants and animals. Animals and plants live in a hostile world. Each day is a search for food, a battle with predators and a constant struggle with the natural environment. They use their circadian timing system to help them anticipate the predictable daily changes in light and dark, hot and cold, wet and dry, levels of ultra-violet radiation and the rest.

The earth’s rotation produces a 24 hour rhythm, but the earth also orbits the sun once a year. It does do tilted at an angle. The result is the regular annual change in daylength, heat, rainfall and humidity that we know as the seasons. And just as organisms need to know the time of day, so they have to know the time of year so as to know when to do what.

In the past decade we have made enormous gains in understanding at the molecular level the clockwork mechanism that drives the circadian rhythm. The basis of this rhythm is an elaborate gene regulatory network that produces cells capable of synthesising proteins whose concentrations vary in a periodic fashion. This periodicity results from a combination of negative feedback and time delays. The circadian system, which can be thought of as a biological system in the same way as the immune system or the endocrine system, produces a near 24 hour that is synchronised to the daily solar cycle by the external light signal, in the case of humans received by non-rod, non-cone photoreceptors ion the eye..

Many organisms also seem to possess a clock with an innate periodicity of about one year and, by analogy with the daily clock, it is called a circannual clock. How it is constructed is unknown but for animals that hibernate or for birds that spend their winters in the equatorial regions away from changes in daylength, it offers a precise calendar.

Humans clearly have circadian rhythms. Most of what happens in our bodies is rhythmic, showing strong day–night differences. Heart beat and blood pressure, liver function, the formation of new cells, body temperature and the production of many hormones all show daily changes. Even our moods and emotions swing in time to a daily rhythm.

Quite how we are seasonally adapted is still largely unknown. But although twenty-first-century medical science has abandoned many of the ancient notions of the importance of seasonal influence on humans and on their health, it is not so easy to dismiss seasonal effects.

Not only are depressive symptoms, for example, clearly seasonally modulated, as in Seasonal Affective Disorder, but even a response to placebo varies with the seasons. The 10-day response rate to placebo in double-blind controlled trials of various anti-depressants has a threefold higher response rate in summer than in winter.

Our immune systems cycle with the seasons, which may well be one reason why we become ill at some times of the year and others. As Hippocrates said in his Aphorisms some 400 years before the birth of Christ, ‘All diseases occur at all seasons of the year, but certain of them are more apt to occur and be exacerbated at certain seasons.’

Humans have broken many links with the natural world. Our food comes pre-packed, our drink pre-bottled and we take pills instead of chewing leaves. Electricity turns our nights into days, and central heating our winters into spring. But go deep into a dark cave without a watch and after a few days out of the sunlight we revert to ancient patterns. Deprived of time cues, our rhythms slowly drift out of alignment with the outside world.

Trouble starts when this alignment breaks down, which may be the problem with Seasonal Affective Disorder (SAD). In Britain about 2-3% of adults suffer from it while a further 10% of have a mild depression known as the winter blues. SAD is perhaps the most common mood disturbance in women of childbearing age in Britain , unremittingly experienced year after year during the six months between the autumnal and vernal equinoxes. Yet exposure to a bright morning light can be of enormous beneficial effect to many sufferers.

Some of the seasonal effects make intuitive sense. Even now, more babies are born in spring than at other times of the year. The effect is not as marked as it was two hundred or more years ago when births tended to peak around the spring equinox, followed by a secondary peak in September and a low during November and December. This `European’ birth pattern is typical of agricultural populations at higher latitudes. It reflects a high frequency of conceptions in June and July and a low conception rate during the autumn harvest season. But even in today’s world the effect is still there.

Other seasonal effects are downright weird. The month you are born can have an effect on how long you live. A study of more than two million Danes and Austrians who died in the last three decades of the twentieth century suggests that, in these two countries at least, lifespan and month of birth are related In Austria, on average those born in the fourth quarter live a little over 200 days longer than those born in the second quarter, whereas in Denmark the difference is about 100 days.

It seems absurd that the month in which you are born can affect your future life chances, but as well as how long you live, how tall you are, how well you do at school and college, your body-mass index as an adult, your blood pressure, the age of menarche for a girl and menopause for a woman, the likelihood of an eating disorder, your fecundity, your likelihood of autism or of panic disorder, your morning versus evening preference and how likely you are to develop a range of diseases, including devastating conditions such as schizophrenia and Alzheiner’s, are all correlated to some extent by the time of year in which you emerged from the womb.

There is an increasing recognition of the impact that changes in the maternal environment can have on later life of the offspring. Environmental conditions are intimately dependent on season, and as we are undoubtedly influenced by such conditions, the foetal or neonatal stages would probably be the most susceptible. It makes sense that the season of birth could be considered as a significant factor for our health and sickness. During the nine months from conception to delivery, the developing foetus is secure in the mother’s womb. But it is not isolated from the world. The foetal physiology is enmeshed with that of the mother and can be challenged during the pregnancy with effects that may not be visible for years to come.

As we unravel these seasonal effects and learn more about how our daily and seasonal rhythms work among ourselves and also in animals and plants, we are beginning to realise that while we try hard to divorce ourselves from the natural world and its patterns of light and dark and lengthening and shortening days we may be damaging our individual and societal health in ways in which we are only just beginning to be aware.

Leon Kreitzman is co-author, with Russell Foster, of Seasons of Life published by Profile Books in June 2009.