What are Circadian Rhythms?

How often do you have difficulty getting up in the morning? Do you sleep later during the school holidays? Despite what your parents may say about your need for sleep, there is actually a biological reason for teens to wake later and it’s all based around your body clock or, more precisely, your circadian rhythms.

The rotations of the Earth around the Sun and the Moon around the Earth produce the cycles that we know as night and day, lunar months and years. Animals and plants have rhythms that match these cycles. A simple example of this is activity linked to day and night. Kiwi are active at night; behaviour that is described as nocturnal. Pūkeko are active during the day; behaviour that is described as diurnal. Daily rhythms such as these are called circadian rhythms.

Circadian rhythms are found in most living things, including plants, animals and many microorganisms. These rhythms are the repeating patterns that we see in the biochemical, physiological and behavioural processes. The rhythms follow a roughly 24-hour cycle linked to the patterns of light and dark in the environment around the organism. Humans are no exception, with processes and behaviours such as sleep, cell division and alertness following this 24-hour cycle. A fascinating fact about circadian rhythms is that when we take away the environmental cues, such as light and dark or the temperature changes that occur during the day and night, the rhythms keep going. If you place a person in a room where there is always light and the temperature never changes, they will still follow a roughly 24-hour pattern of sleeping and waking. This led scientists to suspect that the rhythm is controlled by a biological clock inside the organism.

Human Circadian Rhythms

Circadian rhythms in humans can be investigated by measuring the rhythms of peak performance for physiological and behavioural processes. Figure 1 shows examples of processes in humans that display these rhythms. Notice that, in most cases, there is a period of peak activity or peak performance during the 24-hour cycle. Look at the example of skin cell division (mitosis) which peaks after midnight for about a two hour period.

The time that it takes for a circadian rhythm to run one cycle is called the period of the rhythm. In most organisms the natural or innate period of the circadian rhythm is not exactly 24-hours, however, the regular changes in light and temperature that we experience in our environment help us to adjust our biological clock to a 24-hour day. This called entrainment.

Experiments have clearly demonstrated that the human biological clock ticks with an innate period of 24.3 hours, slightly longer than the 24-hour day. This was measured by putting people into constant environmental conditions where light and temperature did not change at all over a period of several months, allowing their biological clocks to ‘free-run’. Scientists measured when activity started and stopped in the people taking part in the experiment. These measurements are graphed to show when the subjects are active. The graphs of the activity patterns are called actograms. A typical set of results is shown in the actogram in Figure 2.

The actogram in Figure 2 shows the results of activity measurements for one human subject over a period of 87 days (almost 3 months). Look carefully at the X-axis. It covers a period of 48 hours. Each day is shown side by side with the next day so that you can see clearly when the activity patterns start and end.

Knowing that humans have a biological clock gives us a better understanding of what causes jet lag and how we can help alleviate it. Jet lag results from travel so far east or west that our biological clocks are not immediately able to adjust. If you travel to California, for example, it is approximately nineteen hours behind New Zealand time. This means that when you arrive your biological clock is nineteen hours out of sync. Consequently, in the first few days, it can be very hard to get up in the morning because our clocks are adjusted to New Zealand time so are telling our bodies that it’s not time to wake up. Eventually, our biological clock does adjust but we can speed this up by using knowledge of how light affects human biological clocks. By getting light in the morning and avoiding it in the evening we will assist our circadian clocks to ‘phase advance’ to California time. By contrast, getting light in the evening and not in the morning in California will make the jet lag last longer.

Just like travel, puberty and adolescence can also cause jet lag of sorts where teenagers find it very difficult to wake up in the morning. Some scientists call this ‘social jet lag’. Between childhood and adulthood the timing of sleep changes drastically. The diagram in Figure 3 shows the self-reported sleep timing (or chronotype) of people from the ages of five to 90. As you can see, when we are young we wake up early but between the ages of 15 and 20 we start to sleep until later and later in the day. Then, between the ages of 20 and 30, people drift back to sleeping earlier again. These changes cannot be attributed to genetic change as the genes controlling our clocks remain the same throughout life. The differences in waking time appear to result from a combination of behavioural and physiological changes that occur during puberty and adolescence.

As a result of data like this, some high schools around the world (including one in Wellington) have changed school times to starting and ending later in the day. Many are now reporting less students falling asleep in class! One important thing to remember is that these changes in sleep timing are not the result of laziness. If you find that you are struggling to get up in the morning, a good way to combat this is to avoid staying up too late at night. It is also important to avoid high levels of light exposure late in the evening (which will phase delay your clock) and to maximize your light exposure in the early morning (which will advance your clock). So sleep with your curtains open, avoid your phone at night time and go for a morning walk.

The Workings of the Biological Clock 

Animals and plants possess endogenous biological clocks whose innate period is linked closely to the period of the relevant geophysical cycle such as day length or year length. We say a biological rhythm is endogenous if it continues even when external cues such as day length are removed. By observing behaviour and physiological processes, we can clearly see these rhythms. The biological clock that controls circadian rhythms in mammals consists of two groups of nerve cells in the brain called the Suprachiasmatic Nuclei (SCN). The SCN are found in the hypothalamus just above the optic chiasm - the part of the brain where the optic nerves partially cross (Figure 4).

Acknowledgements

Unpublished data shown here was collected by: Dr James Cheeseman, Dr Craig Millar, Ms Eva Winnebeck from the University of Auckland and Professor Randolf Menzel and Mr Konstantin Lehman from the Free-University of Berlin, Germany.

Graphic artwork (Fig 1, 2, 4) produced by Vivian Ward, School of Biological Sciences, University of Auckland.