What is my chronotype?

Your chronotype refers to your body’s natural tendency to wake and sleep at certain times.

Some of us, popularly referred to as “early birds,” “morning larks,” or simply “morning people,” naturally prefer to wake up earlier. Benjamin Franklin, for example, is reputed to have woken up at 5am each morning. This is known as having an early chronotype. People with an early chronotype (i.e. morning people) such as Franklin tend to be more mentally and physically active relatively earlier in the day.

By contrast, others of us, widely dubbed “night owls” or “evening people,” naturally wake up and go to bed later in the day. Such people with a late chronotype (famous examples being Charles Darwin and Winston Churchill) may find themselves more alert and active later in the day.

One approximate way to assess chronotype is to look at the midpoint of sleep. For example, if you slept for 8 hours, having gone to bed at 0000 (midnight) and woken up at 0800, your midpoint time of sleep would be 0400. Midpoint time on work-free days (e.g. the weekend), where you are more likely to sleep and wake up when you naturally prefer, is widely used in research studies as a rough measure of chronotype. People with earlier chronotypes will have earlier midpoints of sleep time, whereas those with evening chronotypes will have later midpoints.

The graphs below are taken from an analysis of 53,689 men and women enrolled in the American Time Use Survey. As can be seen from the distribution of midpoints of sleep, there is considerable variation in chronotype. Furthermore, notice there is a “bell-curve” or Gaussian distribution of chronotypes, with the majority of people having a midpoint of sleep between 0200 and 0400, and with earlier and later chronotypes being less frequently seen.

Source: Fischer, D., Lombardi, D. A., Marucci-Wellman, H., & Roenneberg, T. (2017). Chronotypes in the US–influence of age and sex. PloS one, 12(6), e0178782.

The term "morningness" simply refers to someone's preference for waking up and going to bed earlier in the day, and being active in the mornings over evenings. In other words, "morningness" is the extent to which someone has an early chronotype. (By contrast, "eveningness" describes the extent of havign a late chronotype). Looking at the above graphs, we would expect those with earlier sleep midpoints (left-hand side of the distributions) to have a higher degree of morningness than those with later sleep midpoints.

As we'll find out in later sections, researchers often assess a person's morningness/eveningness by simply asking them a series of questions about their current and preferred sleep habits, such as: "What time would you get up if you were entirely free to plan your day?" or "How easy do you find it get up in the morning?". The responses are then collated to give someone a morningness-eveningness score.

Why do people have different chronotypes?

Why is it that some of us naturally prefer to wake up and go to sleep earlier than others?

Our chronotype is strongly dictated by the timings of our sleep-wake cycle: our 24-hour pattern of alternating sleep and wakefulness.

As described in much greater detail in the Sleep/Wake Cycle trait article, one sleep-wake cycle spans the time from first waking up, to sleeping, and then to waking again. For the average person, a sleep-wake cycle is roughly 24.2 hours, but there are considerable individual differences in the length and timings of our sleep-wake cycles, which underlies why some of us are morning people and others are evening people.

At a neurobiological level, the timings of our sleep-wake cycle are regulated by our body’s master clock, which is located in a part of the brain called the suprachiasmatic nucleus (SCN).

Source: Hickie, I. B., Naismith, S. L., Robillard, R., Scott, E. M., & Hermens, D. F. (2013). Manipulating the sleep-wake cycle and circadian rhythms to improve clinical management of major depression. BMC medicine, 11(1), 1-27.

Virtually all of our cells have their own internal biological clocks, which are responsible for circadian rhythms - biological processes (e.g. hormone production, fat metabolism, body temperature regulation) that follow an intrinsic 24-hour cycle. The master clock in the SCN acts to synchronise these peripheral biological clocks, as well as our sleep cycle, in tune with changes in patterns of light and dark, food intake, and physical activity.

Our genetics also have a strong influence on the intrinsic timings of our master clock. As the master clock regulates the timings of our sleep-wake cycle, gene variants that affect the master clock function can alter our chronotype. Put simply, hundreds of different genes, each exerting a small effect, play a major role in whether we’re more likely to be a morning or evening person.

Source: Carvalho, F. G., Hidalgo, M. P., & Levandovski, R. (2014). Differences in circadian patterns between rural and urban populations: an epidemiological study in countryside. Chronobiology international, 31(3), 442-449.

Of course, chronotype is also a product of social and cultural factors too. Our sleep patterns and preference for mornings or evenings is heavily shaped by our environment (including patterns of exposure to sunlight), social schedules, and lifestyle demands. One study of people in the Vale do Taquiri region of Brazil, for example, found that those living in rural areas were much more likely to be morning people than those in urban areas. This is shown in the bar graphs above.  

What is the HCRTR2 gene?

The HCRTR2 gene encodes a protein called the orexin (hypocretin) receptor type 2 (OX2 receptor).

This receptor is a key component of the brain’s orexin (also known as hypocretin) system, which is involved in various functions, including wakefulness, arousal, sleep, food intake, energy balance, reward, and mood.

- The orexin system

A small structure in our brain called the hypothalamus contains orexin-neurons that produce nerve-signalling molecules called orexin-A and orexin-B.  When these molecules bind to the orexin receptor type 2 on other neurons, they activate them.

Orexin-neurons project from the hypothalamus and connect to wider parts of the brain, including those parts of our cerebral cortex involved in arousal, wakefulness, and regulation of our sleep-wake cycle. Activation of orexin-neurons causes excitation of these brain regions, causing us to feel more awake.

Source: Gotter, A. L., Webber, A. L., Coleman, P. J., Renger, J. J., & Winrow, C. J. (2012). International Union of Basic and Clinical Pharmacology. LXXXVI. Orexin receptor function, nomenclature and pharmacology. Pharmacological reviews, 64(3), 389-420.

For example, as illustrated in the diagram above, orexin-neurons from the hypothalamus can activate histaminergic neurons (HA) neurons in the tuberomammillary nucleus (TMN), which project to cortical areas that promote arousal and wakefulness.

As well as promoting wakefulness, the orexin system is also responsible for regulating when we enter the REM phase of sleep.

- HCRTR2 gene variants

A SNP (single nucleotide polymorphism), designated rs2653349, causes a change G → A change in the DNA code of the HCRTR2 gene.

This gives rise to two different HCRTR2 gene variants:

• ‘G’ variant
• ‘A’ variant

As we’ll describe in the following sections, the ‘A’ variant has been linked to morningness, ease of getting up in the morning, and more frequent daytime napping.

How do HCRTR2 gene variants affect morningness?

Carriers of the ‘A’ variant (rs2653349) of the HCRTR2 are shown to be more likely to prefer mornings over evenings, wake up and go to bed earlier, and find it easier to get up in the morning.

In this respect, a large genome wide association study, encompassing 697,828 white, European individuals enrolled in UK Biobank and 23andMe research cohorts, found that the ‘A’ variant was associated with morningness.

In the study, subjects were asked the following question: “Do you consider yourself to be?:

• definitely a ‘morning’ person
• more a ‘morning’ than ‘evening’ person
• more an 'evening' person
• definitely an 'evening' person
• do not know
• prefer not to answer.

The researchers found that the ‘A’ variant was associated with a 1.07 times higher odds of someone describing themselves as “definitely a morning person” or “more a morning than evening person.”

Subjects in the UK Biobank and 23andMe cohorts were also asked: “On an average day, how easy do you find getting up in the morning?”.

The possible answers included:  “not at all easy”, “not very easy”, “fairly easy” and “very easy”.

‘A’ variant carriers were found to be more likely to find it “fairly easy” or “very easy” to get up in the morning.

- Small effect of HCRTR2 gene

It is worth pointing out that chronotype and morningness are influenced by hundreds of different genes, each of which has a tiny effect. In this respect, the ‘A’ variant of the HCRTR2 gene alone has a very small impact on your sleep patterns.

For example, a portion of the UK Biobank subjects in the study were asked to wear an activity monitor on their wrist for the period of a week so that their sleep patterns could be assessed. Using this data, researchers were able to calculate the midpoint of sleep, which, as we explained earlier, is a rough measure of someone’s chronotype.

Each copy of the ‘A’ variant was associated with a 0.72 minute earlier sleep midpoint. The researchers also looked at the midpoint of the most active 10 hours of a person’s day (known as M10). Each copy of the ‘A’ variant was associated with a 1.38 minute earlier M10 midpoint.

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