The science surrounding chronotypes
is interesting in and of itself. In recent years, we’ve learned that there are
connections between chronotype and personality traits, mental health
risks and overall physical health. However, recent science regarding
the genetic component of chronotype take the scientific understanding
of chronotypes to the next level, opening up the potential to glimpse the
origin of chronotypes as well as to see their value in evolutionary terms. Not
only does chronotype vary between individuals, but it also varies within
a single individual according to age, leading some researchers to theorize
about the evolution of and security value of these variations in
hunter-gatherer cultures.
In the most basic sense of the
word, chronotype describes a person’s natural sleep, wake and
activity rhythm within the approximately 24-hour cycle that makes up a person’s circadian
rhythm. The basic groupings are morningness, eveningness and intermediate. Due
to its role in sleep timing, which can impact circadian rhythm, chronotype can
impact physical and mental health in a variety of ways. That is, in part, due
to the role of the circadian rhythm in the timing of near countless complex
bodily processes, such as the manufacture and release of essential hormones,
including melatonin and insulin. Furthermore, disruptions in the circadian
rhythm, sleep quantity and sleep quality, a common occurrence when chronotype
and lifestyle are at odds, can also negatively impact health and well-being,
increasing the risk of numerous diseases.
Modern science and technology has
opened up vast research potentials, something that has greatly benefited the
study of chronotypes. With the ability to conduct genome-wide association
studies (GWAS) on a massive scale, scientists have been better able to
confirm that chronotypes do have genetic components. There are
identifiable genetic differences between the basic chronotype groups. For
example, a genetic variation in the PERIOD2 (PER2) gene is linked to
later chronotypes, as opposed to those that would fall into the morningness
category.
A study based on the data of about 700,000 people, collected from
23andMe and the UK Biobank, found there to be 351 genetic variants at play in
determining chronotype. This confirms and adds to earlier studies
noting genetic variations, with the newer study revealing far more variations
related to chronotype determination. Studies suggest that between 20 percent
and 50 percent of the factors involved in determining chronotype are present at
birth due to genetics.
Some of the genetic variations that influence chronotype occur in
genes that have to do with the correct functioning of body clocks. Other
chronotype-impacting gene variations were found in the genes associated with
the retina and how it translates light into signals suitable for the brain.
These are just a couple of the genetic variations linked to chronotype.
Evidence of a genetic component to
chronotype opens up new avenues of thought when it comes to the origin of
chronotypes. Viewed through an evolutionary perspective, there could be
practical value associated with chronotype variation among individuals living
in a group. As noted in a study published by the Proceedings of the Royal
Society B: Biological Sciences, sleep can be a vulnerable, dangerous time for
animals, including humans — particularly those in pre-industrial,
non-urbanized, non-agrarian societies.
Thus, variations in chronotype that result in the group not all
sleeping and waking at roughly the same time, but instead result in various
members of the group being awake at different times throughout the night, could
be a real benefit to the security of the group and even enhance survival rates.
On the path of evolution, for the most part, those that survive have a better
chance of passing on their genetic material.
Interestingly, the international team of researchers involved in
the study found that among the hunter-gatherer Hadza of Tanzania, all members
of the group slept at the same time for just 18 minutes. The researchers
observed the group for 20 days. According to the researchers, someone was awake
98.8 percent of the time. That means, in case of danger, someone was available
to wake and warn the group for quite close to the entire night. The researchers
explained that because of the similarities of the Hazda lifestyle to that of
our ancestors in the Pleistocene period, it may be possible to
glimpse some elements of our own evolutionary path.
In contemplating our evolutionary
history, it is worth noting that how our bodies function is the result of
thousands upon thousands of years of evolution. While we who live a modern
lifestyle may feel as though we are far different from our Pleistocene
ancestors or even from the Hazda, the amount of time that we’ve been living
this lifestyle is just a moment in time when looking at the vast expanse of our
history.
Those ancient rhythms still impact how our bodies function today.
It makes sense, then, for better sleep and for better health to work with
our chronotype, arranging our obligations and activities to fit better with our
natural rhythms as much as possible.
https://www.chronobiology.com/the-origin-of-chronotypes-did-sleep-time-preference-evolve-over-time/
