[LEAPSECS] Bio cycles and dueling timescales (was Letters Blogatory)

[Richard Clark]

I'm curious about the repeatability of natural biological cycles. I suspect
that most of them are actually triggered by external nonbiological cues
rather than being 'biological clocks' in our sense of the word clock.

Some that come to mind are annual. The swallows at Capistrano (and the
buzzards at Hinkley). I'm not sure how precisely repeatable they actually
are vs their popular representation.

But one that I have directly observed is the large bat colony under the
Campbell Ave bridge over the Rillito in Tucson. During June (Tucson
doesn't have weather in June) they come out en mass starting 7-9 minutes
after sunset. Very precisely, very repeatably. I imagine the cue is more
the illumination level than an internal clock. Also some sort of social
or aggrigate behavior may be involved. Later in the summer the mass exit
is more diffuse.

Some situations of humans adapting (or perhaps not adapting) to other day
lengths that come to mind, other than shift work in general, that come to
mind are the watch assignments on nuclear subs (repeats after 18 hours on
US subs) and mission control operators for the first few months of a newly
arived Mars lander (24h 39m). In the case of the Mars operators it's
tempting to try to actually adapt to the 24.65 hr day. There can even be
enough of them to form a group with some social identity, overlayed on
their home life. Talk about problems of where to draw the line between two
different timescales! The submariners actually are an isolated social
group but 18 hr isn't a period humans are likely able to adapt to.

What studies of these groups (and maybe South Pole winterovers too) have
there been?

Richard Clark
NSO/NISP
Tucson, Az.


On Wed, 11 Mar 2015, Kevin Birth wrote:

> Solar time is good for humans, but as everyone on this list knows, solar time is not the same as mean time or UTC.
>
> From a chronobiological perspective, mammals have a small cluster of neurons at the base of the hypothalamus called the suprachiasmatic nucleus (SCN).  There are two parts to this structure.  The medial portion has a robust free-running rhythm of around 24 hours plus or minus about 15 minutes.  The two ventral portions connect to the optic nerve and have no strong rhythm.  Instead, the ventral portions work to reset the dorsal part so that the @24 hour rhythm always anticipates the next sunrise regardless of seasonal variations in the length of the daylight period (or the equation of time).  One could say that the SCN is an evolutionary adaptation to Earth's foibles.
>
> The SCN then operates quite differently from representations of solar time, mean or apparent, which chart the rotational day.  In fact, the SCN works much like some old forgotten systems of timekeeping like Bohemian or Italian time, which reset every day.
>
> The features of UTC that we celebrate--continuity, uniformity and standardization--are features that are useful for measuring biological cycles but warp our understanding of those cycles if we begin to think of those cycles as having the same features of uniformity as UTC.  This is true whether or not there are leap seconds.  One of the shortcomings of modern chronobiology and psychophysics of time perception is that as they move more and more into laboratory settings from field settings the cycles are clock controlled, i.e., uniform.  As a result, a lot of current biological science of timing is actually studying how well organisms adapt to humanly created time cycles rather than environmental cycles tied to the Earth's rotation and weather conditions.
>
> Since many human activities are now structured by UTC and not circadian rhythms, many of those activities are, in fact, unhealthy.  In a sense, with regard to what Folkman worries about in his blog, the horse has left the barn and galloped to the border, cleared customs, and now is in another country and most people still don't know the barn door is open much less the horse is gone.  The disconnect between social rhythms, human biology, and apparent solar time began hundreds of years ago when preference in timekeeping shifted in favor of mean time and 24 hour days beginning at midnight, and this disconnect has been exacerbated by artificial lighting.
>
> Cheers,
>
> Kevin
>