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The reason your dog knows you are coming home!

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Have you ever been in the house waiting for your father or husband to come home and about 10 minutes before he walks in, the dog seems to instinctively know he is on his way? Or do you hate waking early at "work time" on a Saturday although the alarm clock has not been set and you had gone to bed later and really wanted to sleep in? 

Magicicada periodical cicadas— the world’s longest-living insects —lie dormant underground for 13 to 17 years before tunnelling out to feed and breed. Interestingly, they have three small eyes, or ocelli, located on the top of their head between their two large eyes. MARIANO SZKLANNY

These are two examples of biorhythms, which are what you could call the actions of your body clock. Biorhythms are not to be confused with the pseudoscientific practice of the same term where your physical, emotional and mental activities are apparently affected due to biological rhythms and where predictions connected with aspects of your life can be deduced.

Please be aware that the latter has a scientific basis similar to that of astrology, intelligent design, witchcraft, divination or paranormal activity. That is, it does not stand up to rigorous scientific testing and has no scientific basis. It is either based on guesswork or interpretations without the benefit of scientific methodology.

The science of biorhythms that I want to discuss is now referred to as chronobiology - the timings in the biological activity of living organisms. It is easier to assume that these biological activities at certain times are specific to animals, for which foraging, mating and, of course, sleeping are all timed functions, but the rest of life on earth is also subject to rhythms.

Plants are hugely dependent on the variations and timings of the world, especially since they photosynthesize - an activity that can only happen during the day. Fungi, protists and even bacteria have been observed living according to rhythms, be they daily in accordance with the sun, monthly being dependent on lunar cycles or yearly as a result of the seasons.

Circadian rhythms

The circadian rhythm is arguably the most important biological rhythm in nature, and it takes place over about a day, which is its literal meaning. The biggest factor in the circadian rhythm is the day/night, light/dark cycle, and although many organisms have a built-in mechanism for dealing with daily rhythms (instinctive you would say in animals), these rhythms can be altered by environmental factors.

An animal's behaviour can be described as diurnal, meaning that the animal is active in the day, or nocturnal, which means that the animal is active at night. Whether an animal is diurnal or nocturnal is governed by many factors, including genetics and the biochemistry of its metabolic processes. This phenomenon is also dependent on the animal's physiology. Many animals have specific adaptations, depending on their being diurnal or nocturnal.

During a total solar eclipse, some animals have been observed to be responsive to the twilight imitation of totality. Some diurnal birds and reptiles exhibit behaviours similar to how they would act if night was falling.

Some nocturnal frogs and owls have been observed calling as they would if night was coming soon, but on the whole, most animals don't react much differently during a solar eclipse. This strongly suggests that the daily rhythm is internally controlled as opposed to being purely a light/dark response.

Jet lag

This behavioural symptom is known technically as desynchronosis. Anyone who has travelled across many of the earth's meridians (which ultimately dictate time zones) in a short amount of time may have experienced jet lag.

When you undertake transmeridian travel, your sleep/wake patterns are disrupted. Your sleep/wake patterns are one of your circadian rhythms that are very much in tune with the day/night, light/dark experiences of everyday life, and jet lag contradicts those patterns.

Your body needs to resynchronise this circadian rhythm to be in tune with your daily activities, but it takes time. Meanwhile, you remain jet lagged. Eating times, temperature, hormone regulation and obviously sleep times are no longer in tune with the environment, and this can lead to problems.

Feeling easily irritated, entering a sparky mood, going into a slump, insomnia and diarrhoea are all symptoms of jet lag. You can speed up the recovery from jet lag by eating at "local" times, getting out in daylight and staying awake, fighting the urge to sleep during daytime. So, although circadian rhythms are "inherent", they can be managed and altered.

Lunar rhythms

Lunar rhythms such as those of organisms' responses to the moon are not what I mean. Despite many people being convinced that a full moon alters your behaviour, that position is not supported by scientific evidence.

The moon has a huge effect on the earth, especially with regard to the tides. And if you are a tidal creature, you have to be in sync with tidal cycles. Generally, there are two high and two low tides in a day. Should you be a coastal animal or a coral reef dweller, the moon and its tidal results are essential to your reproductive success.

Corals are a prime example of this situation. Most species of corals release sperm and eggs at certain times of the year when tides are at their highest and currents are slowest. This allows corals to substantially increase their reproductive success. Without these lunar timings, it would be difficult for corals to reproduce.

With humans, a monthly cycle is the menstruation cycle, although this cycle has nothing to do with lunar cycles. It certainly is a rhythm, though, and, indeed, it is a biorhythm that has profound effects on human life.

Circannual rhythms

Similarly to how circadian rhythms are about a day in length, circannual rhythms are about a year in length. Many breeding cycles experienced by animals are based on a circannual rhythm. This has essentially to do with the effects of the seasons, mainly their daylight hours and mean temperatures.

Many birds migrate for winter, and often do so in colonies of hundreds of thousands of birds. An entire colony may begin migration within a week. Antler growth and shedding experienced by many hoofed mammals are also conducted circannually, as is flowering by many plants.

Pandas have a miniscule window for breeding every year, but they are not alone, with many breeding cycles of many animals being on a circannual rhythm. Hibernation is also a circannual rhythm that is essential to the survival of many species, such as bears.

So, basically, life is all about clocks. I have only mentioned clocks that are logical to us, but there are bizarre examples, such as cicadas that are on a 13- or 17- year life cycle from egg to adulthood, but that is a biorhythm all on its own!

Although there must be some logic to the cicadas' clock, all of the aforementioned biological rhythms make perfect sense when you think about it. The daily, lunar and seasonal changes create dynamic conditions for life on earth, so not being affected by them or not getting "in tune" with them would certainly not bode well for survival.

Dave Canavan has an MSc in Behavioural Ecology and is the principal of Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You may contact Dave at

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