Ultimately, the self-invented week, including the holy weekends, has come to rule modern life as compellingly as the alternation of night and day and the coming and going of the seasons. It was made even more clear by the strange variation in registered corona infections that is associated with the reduced testing on Saturday and Sunday. On closer inspection, it is teeming with weekend effects: in purchasing behaviour, drinking, accidents, hospitalizations, you name it. Their influence can have a far-reaching effect, for example in air pollution and, by extension, cloud formation and rain. In the summer months in the US there is more rain in the middle of the week than on the weekend. In many places on Earth, a weekly cycle has been observed in the diurnal temperature variations.

The alternation of working days and days off has existed for so long that most people have deeply absorbed the weekly rhythm. This is how the Kortjakje syndrome arose: sick reports predominate on Mondays and early in the week the word ‘health’ is more often Googled than on Saturdays and Sundays. If you enter the search term ‘circaseptan rhythm’ at Google Scholar, you will also find the more hidden human weekly rhythms such as variations in the blood count, urine composition and hormone balance. You didn’t know you had it in you.

It is conceivable that many pets now have their own weekly rhythm, but science is silent about this. Cats and dogs probably react directly to the rhythm of the housemates.

Spring tide and dead tide

Strangely enough, animals – and even plants – have their own weekly rhythms in the wild, far from human bustle. The magazine Chronobiology International gave an overview in 2016. You can find them in rats, fish, flies, springtails, beans and all kinds of small sea organisms. Nobody knows what evokes the rhythms, nobody knows what the pacemaker is.

Special attention was paid in 1998 to a weekly rhythm with the New Zealand beach beetle Chaerodes trachyscelides that lives on incrustations. In this case, the researchers believed they could explain the rhythm: the wash-up sorts on the beach in a pattern that follows the weekly alternation of spring tides and neap tides. It has led to the assumption that some of the weekly rhythms in humans also originated in such a way and are therefore evolutionarily ancient. Perhaps primitive man also rummaged for the wash-up on the beach.

Oh, how the amateur researcher would also like to discover a hidden rhythm for himself. First find the regularity, then prove that it is statistically significant, then the pacemaker and finally formulate a specific benefit.

The research on lunar rhythm is older than the research into weekly rhythms: lunar rhythms. That too has expanded. Half a century ago, examples were only the disgusting nocturnal release – at the moon in the last quarter of an hour – of sperm and eggs by the palolo worm (Palola green) and the striking reproductive rhythm of the oyster (Ostrea edulis). The latter was investigated by fisheries biologist Piet Korringa. He established that oyster larvae always swim out of the shell ten days after full moon and new moon.

Measurably small

More animals and plants have since been found that show a rhythm of 29.5 days or that appear to be under the influence of the moon in some other way. The Wikipedia entry ‘lunar effect’ provides an overview. Often it concerns the reproductive cycle or the hormone balance. How the moon exerts its influence is rarely visible. The light she emits at night is dim. Although the effect of its gravity on the tides is great, biologists usually overlook that the influence on individual organisms with their small mass is immeasurably small. Recently, much has been expected of a lunar influence on the Earth’s magnetosphere.

But rarely are the well-thought-out statistics convincing. When in 1980 it was ‘shown’ that the human menstrual cycle is influenced by the moon, as had been suspected for centuries, only very young students with a cycle of exactly 29.5 days were looked at. In 47 of the 68 girls, menstruation started in the week before or after the full moon.

Two months ago, in Science Advances once again ‘demonstrated’ an influence of the moon. 22 women had been tracking details of their cycles for years and from time to time did the events really keep pace with the synodic lunar movement or a Others lunar cycle because there are more. Voila. This close to home, the amateur researcher can get to work.