The chronotypes

Feeling like heading to bed at 9 pm and waking up at the crack of dawn? These tendencies define your chronotype, backed up by changes within your body. A generally overlooked topic, chronotypes affect our everyday behaviour. Many people innately associate themselves with a certain chronotype, but what do we know about how these physiological differences are caused at a molecular level? 

The word ‘chronotype’  was first coined in the 1970s, combining the Greek words chrono (time) and type (kind or form). While the term is relatively modern, the concept emerged in the 18th century. Researchers in the 1960s and 1970s, like Jürgen Aschoff, explored how internal biological clocks influence our sleep-wake cycles, leading to the classification of people into morning or evening types based on their activity patterns. The first evidence of body clocks was found in plants rather than humans, thus leading to the invention of flower clocks, which were used to tell the time of the day.

Before delving into the details, let us be introduced to the general categories of chronotypes, which describe a person’s inclination to wake up and sleep while also affecting productivity periods. We know of the following three categories:

• The morning type (also referred to as larks): they are inclined to wake up and go to bed early because they feel most alert and productive in the mornings.

• The evening type (also called the owls): they feel most alert and productive in the evenings and onwards, so they are inclined to wake up and go to bed later.

• The intermediate types (also referred to as the doves): they fall in the middle of this range. 

  
  

Let’s explore what we know about the genetics that prove that chronotypes are a natural phenomenon.

Genetics of chronotypes

The main determining factor in our chronotypes is the circadian period. This is the body’s 24 hour cycle of changes that manifest into feelings of productivity and energy or tiredness. The length of this is crucial in determining our chronotypes. More importantly, specific physiological changes that cause these effects are melatonin and core body temperature.

One study suggested that the morning types might have circadian periods shorter than 24 hours, whereas evening chronotypes might have circadian periods longer than 24 hours.

A major clock gene is PER, which includes a collection of genes known as PER1, PER2 and PER3, which are thought to regulate circadian period factors. 

Specifically, it has been observed that a delay in the expression of the PER1 gene in humans causes an increased circadian period. Possible causes for this delay may be rendered to a variation within the negative feedback loop that PER1 operates in, including hereditary differences, environmental causes, changes to hormonal signals and age. This process may describe the mechanism behind the evening chronotype. 

Molecular polymorphs in the PER3 gene are thought to cause shorter circadian rhythms and the manifestation of the morning types. Similarly, a polymorph in the PER3 gene can be caused by a plethora of causes, as described for PER1. These nuances cause differences in the periodic release and stop of hormones which control the circadian rhythm, such as melatonin and body temperature. This is important in its power to control our energy levels, windows of productivity, and sleep cycles.

The consensus remains that chronotypes are attributable to genetic premeditation by 50%, however, it has also been observed that chronotypes are prone to change with advancing age. Increased age is associated with an inclination towards an earlier phase chronotype. Age-related variation has been observed to be higher in men. There also exists an association between geographical locations and phase preference; increasing latitude (travelling North or South) from the earth's equator is associated with later chronotypes. Of course, many variations and factors come into play to affect these findings, such as ethnic genetics, climate, work culture and even population density.

The effect on core body temperature and melatonin

Polymorphisms in the PER3 cause a much earlier peak in body temperature and melatonin in the morning than in the evening and intermediate types. These manifest as the need to sleep much earlier in the morning and a decreased feeling of productivity later in the day. In contrast, the evening types experience a later release of melatonin and a drop in core body temperature, causing a later onset of tiredness and lack of energy. It can then be inferred that the intermediate types are affected by the expression of these genes in a way that falls in the middle of this spectrum. 

Conclusion

Understanding differences in circadian rhythms and sleep-wake preferences offers valuable insights into human behaviour and health. Chronotypes influence various aspects of daily life, including sleep patterns and quality, cognitive performance and susceptibility to specific health conditions, including sleep-wake conditions. An extreme deviation in circadian rhythms and sleep cycles may lead to such conditions as Advanced sleep-wake phase Disorder (ASPD) and Delayed sleep-wake phase Disorder (DSPD). Recognising these variations is also helpful in optimising work schedules and aligning to jet lags, improving mental and physical health by tailoring our biological rhythms to our environments. Many individuals opt to do a sleep study at an institution to gain insights into their circadian rhythms. A healthcare professional may also prescribe this if they suspect you have a circadian disturbance such as insomnia.

The Morning-Eveningness Questionnaire (MEQ)

The MEQ is a self-reported questionnaire you may complete to gain more insight into your chronotype category. Clinical psychologist Micheal Breus created it and uses different animals to categorise the chronotypes further. The framework suggests that the Bear represents individuals whose energy patterns are entrained to the rising and the sun's setting and are the most common types in the general population. The Lions describe the early risers, and Wolves roughly equate to the evening types. Recently, a fourth chronotype has been proposed: the Dolphin, whose responses to the questionnaire suggest that they switch between modes.

Whether you're a Bear, Lion, Wolf, or Dolphin, understanding your chronotype can be a game-changer in optimising your daily routine. So, what’s your chronotype—and how can you start working with your body’s natural rhythms to unlock your full potential? A sleep study? The MEQ? Maybe keeping a tracker.

Written by B. Esfandyare

REFERENCES

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Ujma, P.P. and Kirkegaard, E.O.W. (2021). The overlapping geography of cognitive ability and chronotype. PsyCh Journal, 10(5), pp.834–846. doi:https://doi.org/10.1002/pchj.477.

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