The chronotype is a measure of where the internal clock of a person lies relatively to the external time (clock time) [RPZW19].
In common tongue, chronotypes are known as owls (late chronotypes) and larks (early chronotypes). Since sleep is one of the most obvious outputs of the internal clock, sleep phases are most commonly used to determine the chronotype. Alternatively, the time dependent level of alertness can be considered.
Chronotypes are nearly normally distributed with slight surplus of late chronotypes [FLMR00].
Figure 1 shows a distribution of chronotypes using the sleep phases without external restrictions (alarm clock). The colors encode the respective chronotypes. For representation it is used either sleep time (time of falling asleep to time of waking up, scale on the top) or midpoint of sleep (mid-point between falling asleep and waking up, scale on the bottom). The representation via midpoint of sleep has the advantage that it is independent of the individuals sleep duration. When using sleep a sleep duration of 8 hours is uniformly assumed. However, how many hours of sleep a person needs per night is independent of his chronotype. The chronotype only defines when someone sleeps, not how long. Late chronotypes do not sleep longer than early ones, they only sleep later.
Figure 1: Chronotypes. Source: [RPZW19], edited.
Most people accumulate a sleep deficit during the work week by using an alarm clock. This sleep deficit is compensated on the weekend by "oversleeping " [RPZW19]. That means people sleep more hours than they would normally need on average per night. The later the chronotype, the greater the weekly sleep deficit, and the longer the oversleeping on the weekend [RPZW19]. Only very early chronotypes are spared from that. They often even sleep less on weekends due to social constraints (e. g. meeting friends) than during the working week [RKJK07].
The distribution on the right in Figure 1 shows the sleep times on weekends considering oversleeping. By subtracting the effect of oversleeping, you get the distribution in Figure 1 on the left. This shows the sleep times without external constraint and without previous sleep deficit.
Chronotype and social jetlag correlate significantly [WDMR06]. Therefore, late chronotypes are more affected by health consequences of social jetlag than others [Part15]. In most cases, this is not due to the late chronotype per se, but to the time constraints that are inappropriate for the chronotype [LDFC11, PKLR18, Roen19].
Why so different chronotypes?
The internal clock is set by light and dark. Due to genetic differences, however, different people's clocks set differently early or late within comparable light conditions [Roen19]
Age and gender
The chronotype of a person depends on age and gender (see Fig. 2), with men being on average later chronotypes than women [FLMR00, RKJK07, SKAH20].
On average, children are rather early chronotypes and become increasingly late with the onset of puberty [FLMR00, RKPR04]. The maximum is reached with 21 years in men and 19.5 years in women [RKJK07]. This maximum is also understood as a biological sign for the end of puberty [RKPR04]. Then the chronotype becomes earlier again in the course of life [FLMR00, RKJK07, RKPR04].
The phenomenon that teenagers and young adults are later chronotypes, is also observed in animals [HaLe12] and pre-industrial cultures [SCMM17] .
Figure 2: Midpoint of sleep (mid-point between falling asleep and waking up) without restriction and previous sleep deficit. White: Men, black: Women, gray: average of both genders. Source: [RKJK07].
Due to industrialization and the associated light pollution as well as an increasing proportion of indoor jobs, distributions of chronotypes are now wider and later than in times of our ancestors [RKJK07]. While in the pre-electric age owls (late chronotypes) and larks (early chronotypes) were two to five hours apart [SCMM17], today it is 12 hours in urbanized regions [RKJK07].
For the same reason, chronotype distributions are later in urban areas than in rural ones [BoPK10, CaHL14, SKAH20].
Chronotype distributions are usually made from groups of people spread across geographical areas of some extent. Depending on the longitude and latitude, people experience other sun times (times of sunrise and sunset). Sunrise and sunset are later in the west of a time zone than in the east, which results in later chronotypes in the west [Bori10, HEGR14, RoKM07]. In addition, the day length in the north varies more with the season within one year than in the south (in the northern hemisphere). This also influences the internal clock and thus the chronotype [Bori10, RoKM07] (see also season).
The chronotype is also subjected to seasonal effects: In winter, people's internal clocks are on average later than in summer [HBWC18, HBWH14, HNST18, KJMR07]. This is due to the shorter day length. The following applies: The longer the sun day, the earlier the average chronotype [ATKP14, Bori11].
For a well-set internal clock, the body needs a lot of daylight. Unfortunately, in our modern society, we spend most of our days in buildings, often due to professional reasons. We also reduce the natural darkness with artificial light. Even when we switch off the light in our apartment, we perceive the increased artificial light in cities. Both weaken the zeitgeber for our internal clock. Therefore, the internal clocks of most people are nowadays delayed within the light-dark-cycle. The few exceptions are the early chronotypes (larks) that even become earlier due to the weakened zeitgeber. [Roen19]
To set our clock earlier, we especially need a lot of light in the morning. With light in the evening, our internal clock is delayed even further. [RoDM03]
Spend as much time outdoors as possible.
In particular, enjoy light in the morning, less in the evening.
Use as little artificial light as possible in the evening. In particular, avoid light with a high proportion of blue light (e.g. cell phone, computer).
Larks, people who wake up very early, can set their internal clock later by also spending a lot of time outdoors, but enjoying a lot of light in the evening rather than in the morning.
Determination of the chronotype
Since the body's internal clock actually consists of a large number of clocks, it is not possible to measure “the” internal time of the body. [RPZW19] However, it can be estimated by using questionnaires as well as by measuring different biomarkers. Both approaches are used to determine the chronotype. Since measuring biomarkers is very costly and questionnaires provide comparably accurate results [RPZW19], studies that require a large number of subjects often use questionnaires.
Questionnaire: Munique Chronotype Questionnaire (MCTQ)
Der The Munique Chronotype Questionnaire asks 17 questions about sleep behavior on work days and on work free days. It is taken into account in each case ...
when you go to bed.
when you turn off the lights, meaning when you're ready to fall asleep.
how long it actually takes you to fall asleep (sleep latency).
when you wake up.
when you actually get up.
From this data the midpoint sleep of the natural sleep phase (without external constraint and without compensatory sleep due to previous sleep deficit) is calculated [RPZW19]. The midpoint of the natural sleep phase is one of the most precise markers of the internal clock [TTLC01].
There is a specialized questionnaire for shift workers: MCTQ Shift [Them00].
Questionaire: Morning-Evening-Questionaire (MEQ) [HoÖs76]
Biomarker: Dim Light Melatonin Onset (DLMO)
The sleep hormone melatonin is a measure of the body's internal time. The DLMO uses blood, saliva or urine tests to determine when the body begins to release melatonin in a dark phase.
Biomarker: Measurement of cortisol levels
Measurement of activity phases