Paul Rice

@GettingWell4

I publish Take responsibility for your own one precious life.

Joined July 2011

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Zaid K. Dahhaj

@zaidkdahhaj

4 days ago

For the most part, there is only one human chronotype, and it rises with the sun and sleeps relatively shortly after nightfall People’s minds have been hijacked through marketing to believe that a chronotype is a personality trait. That they’re a “bear” or a “dolphin” This is New Age pseudoscientific bullshit A chronotype is simply the timing of when your body naturally wants to sleep and wake, based on how it’s currently synced to light and darkness. It’s the time of day your body clocks are set to under current environmental conditions The key words to remember are “under current environmental conditions” as we discuss artificial light and circadian disruption Studies on hunter-gatherer populations with no artificial light show sleep timing clustering within about a 2 hour window across all individuals 2 hours The 4 to 6 hour spread we see in modern populations is not a matter of genetic expression. A “night owl” doesn’t exist. That is the result of circadian disruption The genetic variation in chronotype is real but modest, and it lives almost entirely in a handful of clock genes The core ones are PER1, PER2, PER3, CLOCK, and CRY1. These genes encode the proteins that drive the transcription-translation feedback loop sitting at the heart of every cell’s 24 hour oscillator. Small mutations or polymorphisms in these genes can slightly lengthen or shorten the intrinsic period of that loop, which in turn shifts the phase of the whole system earlier or later The most studied example is a mutation in CRY1 that extends the intrinsic clock period to around 24.5 hours instead of the normal 24.2. That half hour difference compounds over days and pulls sleep timing progressively later People carrying this variant have what clinicians call Familial Delayed Sleep Phase Disorder. It’s heritable, it’s real, and it genuinely resists behavioral correction. But, people need to understand that this is a disorder. It sits at the extreme tail of the distribution PER3 has a well documented length polymorphism, meaning some people carry a longer version of the gene and some carry a shorter one. The longer variant correlates with earlier sleep timing and stronger homeostatic sleep pressure, meaning those people feel the cognitive consequences of sleep deprivation harder and faster. The shorter variant correlates loosely with later sleep timing and slightly more resilience to sleep loss. This is probably the most relevant polymorphism for what people casually call chronotype The CLOCK gene has a variant associated with evening preference and is also linked to higher rates of mood disorders and metabolic dysfunction, which itself tells you something important, that evening shifted genetics carry health costs, which suggests the ancestral optimal is morning anchored Now here’s what ties it all together from a circadian standpoint: These variants do NOT change what your circadian clocks responds to. Light still entrains every genotype. The variants just shift the default period slightly, so a genuinely late running clock needs a stronger morning light signal to land at the same phase as a neutral genotype. Same prescription, but higher dose Most people claiming a genetic chronotype have never tested that under ancestral circadian aligned light conditions Hunter-gatherer populations carry the same genetic diversity as industrialized ones, but show dramatically narrower sleep timing variance. The genes are there, but the environment just stops amplifying them

zaidkdahhaj's tweet photo. For the most part, there is only one human chronotype, and it rises with the sun and sleeps relatively shortly after nightfall

People’s minds have been hijacked through marketing to believe that a chronotype is a personality trait. That they’re a “bear” or a “dolphin”

This is New Age pseudoscientific bullshit

A chronotype is simply the timing of when your body naturally wants to sleep and wake, based on how it’s currently synced to light and darkness. It’s the time of day your body clocks are set to under current environmental conditions

The key words to remember are “under current environmental conditions” as we discuss artificial light and circadian disruption

Studies on hunter-gatherer populations with no artificial light show sleep timing clustering within about a 2 hour window across all individuals

2 hours

The 4 to 6 hour spread we see in modern populations is not a matter of genetic expression. A “night owl” doesn’t exist. That is the result of circadian disruption

The genetic variation in chronotype is real but modest, and it lives almost entirely in a handful of clock genes

The core ones are PER1, PER2, PER3, CLOCK, and CRY1. These genes encode the proteins that drive the transcription-translation feedback loop sitting at the heart of every cell’s 24 hour oscillator. Small mutations or polymorphisms in these genes can slightly lengthen or shorten the intrinsic period of that loop, which in turn shifts the phase of the whole system earlier or later

The most studied example is a mutation in CRY1 that extends the intrinsic clock period to around 24.5 hours instead of the normal 24.2. That half hour difference compounds over days and pulls sleep timing progressively later

People carrying this variant have what clinicians call Familial Delayed Sleep Phase Disorder. It’s heritable, it’s real, and it genuinely resists behavioral correction. But, people need to understand that this is a disorder. It sits at the extreme tail of the distribution

PER3 has a well documented length polymorphism, meaning some people carry a longer version of the gene and some carry a shorter one. The longer variant correlates with earlier sleep timing and stronger homeostatic sleep pressure, meaning those people feel the cognitive consequences of sleep deprivation harder and faster. The shorter variant correlates loosely with later sleep timing and slightly more resilience to sleep loss. This is probably the most relevant polymorphism for what people casually call chronotype

The CLOCK gene has a variant associated with evening preference and is also linked to higher rates of mood disorders and metabolic dysfunction, which itself tells you something important, that evening shifted genetics carry health costs, which suggests the ancestral optimal is morning anchored

Now here’s what ties it all together from a circadian standpoint:

These variants do NOT change what your circadian clocks responds to. Light still entrains every genotype. The variants just shift the default period slightly, so a genuinely late running clock needs a stronger morning light signal to land at the same phase as a neutral genotype. Same prescription, but higher dose

Most people claiming a genetic chronotype have never tested that under ancestral circadian aligned light conditions

Hunter-gatherer populations carry the same genetic diversity as industrialized ones, but show dramatically narrower sleep timing variance. The genes are there, but the environment just stops amplifying them

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