A Blog Around The Clock

Sun Time is the Real Time

Blogging on Peer-Reviewed Research

If you really read this blog “for the articles”, especially the chronobiology articles, you are aware that the light-dark cycle is the most powerful environmental cue entraining circadian clocks. But it is not the only one. Clocks can also be entrained by a host of other (“non-photic”) cues, e.g., scheduled meals, scheduled exercise, daily dose of melatonin, etc.

Clocks in heterothermic (“cold-blooded”) animals can also entrain to temperature cycles. Lizards can entrain to temperature cycles (pdf) in which the difference between nightime low and daytime high temperatures is as small as 2 degrees Celsius. When taken out of a warm-blooded animal, the SCN clock can also be entrained (if you are a regular here, you recognize the name, don’t you) by temperature cycles (presumably a nice feedback loop that stabilizes the mammalian rhythms: the clock entrains body temperature cycles and body temperature cycles entrain the clock).

Some rodents can phase-shift (and thus presumably entrain if presented daily) their clocks under the influence of conspecifics odors or pheromones. In an old study (which was not very good, but enough can be concluded from the data), rats held in groups in constant conditions entrained their rhythms to each other (while the quail did not), suggesting some kind of social entrainment, perhaps mediated by smell.

Social animals are supposed to be sensitive to social cues and it is presumed that their clocks can be entrained by social cues as well. It is also widely believed that no other animal’s clock is as sensitive to social cues as the human’s.

Everyone who’s been in this field has heard the anecdotes about the experiments conducted by Jurgen Asschoff and others at Andechs, Germany in the 1950s and 60s, in which human volunteers were kept in constant light conditions for prolonged periods of time in old underground bunkers (I think Asschoff’s bunkers are now preserved as monuments to science, just like the Knut Schmidt-Nielsen’s camel chamber is preserved over at Duke University with a nice brass plaque). According to the lore of the field (were those things ever published?), social cues like newspapers, or physical appearance of technicians called in to bring in the food (e.g., sleepy look, or the 5-o-clock stubble) were sufficient cues to entrain human subjects.

It is always difficult to directly test the relative importance of different environmental cues. Sure, one can put them in direct competition by having, for instance, a light-dark cycle and a temperature cycle being 180 degrees out of phase and see to which one of those animals actually entrain (such a study in Neurospora was published a few years back). But, how do you know that the intensities are equivalent? What is the equivalent of 1000 lux in degrees Celsius? Ten, twenty, a hundred?

So, perhaps one should look at the ecologicaly relevant levels of intensity of environmental cues. But how does one dissociate two synchronous cues out in nature in order to do the experiment? Well, of course, use humans for this experiment as the society has already made sure some cues get dissociated! And that is exactly what Till Roenneberg, C. Jairaj Kumar and Martha Merrow did in a new paper in Current Biology:

The human circadian clock entrains to sun time (Volume 17, Issue 2 , 23 January 2007, Pages R44-R45)

What they did is take advantage of the fact that time zones are very broad – about 15 angle degrees each. This means that the official (social) midnight and the real (geophysical) midnight conicide only in a very narrow strip running smack through the middle of the time zone. Most of Europe is one time zone. If it is officially midnight in Europe, i.e., the clock strikes 12, it is really midnight (as in “Mid-Night”) in a place like Munich, but it is already something like an hour later in Bucarest, and still something like an hour to wait for it in Lisbon.

So, in this paper, they looked at actual entrainment patterns of more than 21000 Germans to see if they entrain to the real midnight – suggesting that light cues are stronger, or to official midnight, suggesting that social cues are stronger. They controlled for age, sex, chronotype (owls/larks) and general culture (former East and West Germanies) and what they found was very interesting: in small cities, towns and villages, people entrain to the light-dark cycles and mostly ignore the official time. However, bigger the city, more independent the entrainment was from the real light-dark cycle. The phase was delayed and more in sync with the official time.

It is hard to interpret the findings, really. Do people in big cities entrain to official time due to stronger social cues (the busy big-city life and social scene) or because they are better sheltered from the natural light-dark cycle and, due to all the light pollution and technology, better able to impose on their clocks an artifical light-dark cycle. I am assuming that untangling this question is going to be their next project.

But, one thing this study did was make us take a more skeptical look at all those Andech bunkers anecdotes. Sure, social cues may work in the absence of all other cuse, but they are not THAT powerful and do not seem to be able to overcome the effects of natural light cycles in places in which people are able to percieve a natural light cycle. I guess one can view the life in a big city (“black box”) as being in a laboratory experiment in which the society acts as an experimenter, imposing the light-dark cycle on people, while the life out in the country is more like a field experiment in which the human subjects are exposed to the natural environmental cues.

Addendum

Comments

  1. #1 Markk
    January 31, 2007

    “But, one thing this study did was make us take a more skeptical look at all those Andech bunkers anecdotes. Sure, social cues may work in the absence of all other cues”.

    I don’t get this comment, it seems to me that the bunker studies weren’t invalidated at all, don’t they support the idea given here? That is, where sunlight is highly observable, it dominates, but the less it is visible (zero in the bunker), or the less it is as a percent of the brightness (that is other lights dominate in big cities or people are inside), the more social cues become important.

  2. #2 coturnix
    January 31, 2007

    Sure, they were valid studies, but the anecdotes surrounding them (those obeservations never made it into papers, I believe – the papers were about effects of light and such) turned into legends over the decades, giving an impression that humans are soooooo susceptible to the subtlests social cues. This deflates it a little bit and makes us think instead of just enjoying and promulgating the legends.

  3. #3 Leon Kreitzman
    February 2, 2007

    As I recall reading, in the early days of the Aschoff experiments the regimen involved a gong sounding when the researchers changed from a light to a dark cycle and wanted the subjects to provide urine samples or carry out performance tests. One day the auditory signal failed and although the change from light to dark went ahead, the subjects surprisingly went into a free-running rhythm. They later told Achoff that to them the gong was social contact with the investigators and subsequent experiments led him to believe that in humans the social cues were the prinicpal synchronisers.

  4. #4 Leon Kreitzman
    February 2, 2007

    For some reason this bit did not go with the post

    Czeisler visited Aschoff in 1976 and he was troubled by the fact that the experimenters did not think that switching the low intensity lighting on and off would affect the clock.As Czeisler said ” They didn’t think it would have any effect, but switching on electric lights resets the biological clock. It’s the same as resetting your watch”
    Mind you, it took him years to show that the human rhythm is not a near 25 hours, as Aschoff thought,but 24 hours 11 minutes on average.

  5. #5 coturnix
    February 2, 2007

    Thank you for this – it is nice to hear inside information from people ‘in the know’. We need to collect all this lore in one place for future generations…

  6. #6 Jonathan Vos Post
    February 3, 2007

    Brain time is the real time.

    http://www.sciencedaily.com/releases/2007/01/070131135525.htm

    Source: University of California – Los Angeles
    Date: February 1, 2007

    How Does Your Brain Tell Time? Study Challenges Theory Of Inner Clock

    Science Daily — “Time” is the most popular noun in the English language, yet how would we tell time if we didn’t have access to the plethora of watches, clocks and cell phones at our disposal?

    [graphic: The changing colors reflect how a brain cell network evolves over time in response to stimuli. (Credit: Buonomano Lab)]

    For decades, scientists have believed that the brain possesses an internal clock that allows it to keep track of time. Now a UCLA study in the Feb. 1 edition of Neuron proposes a new model in which a series of physical changes to the brain’s cells helps the organ to monitor the passage of time.

    “The value of this research lies in understanding how the brain works,” said Dean Buonomano, associate professor of neurobiology and psychiatry at the David Geffen School of Medicine at UCLA and a member of the university’s Brain Research Institute. “Many complex human behaviors — from understanding speech to playing catch to performing music — rely on the brain’s ability to accurately tell time. Yet no one knows how the brain does it.”

    The most popular theory assumes that a clock-like mechanism — which generates and counts regular fixed movements — underlies timing in the brain. In contrast, Buonomano suggests a physical model that operates without using a clock. He offers an analogy to explain how it works.

    “If you toss a pebble into a lake,” he explained, “the ripples of water produced by the pebble’s impact act like a signature of the pebble’s entry time. The farther the ripples travel the more time has passed.

    “We propose that a similar process takes place in the brain that allows it to track time,” he added. “Every time the brain processes a sensory event, such as a sound or flash of light, it triggers a cascade of reactions between brain cells and their connections. Each reaction leaves a signature that enables the brain-cell network to encode time.”

    The UCLA team used a computer model to test this theory. By simulating a network of interconnected brain cells in which each connection changed over time in response to stimuli, they were able to show that the network could tell time.

    Their simulations indicated that a specific event is encoded within the context of events that precede it. In other words, if one could measure the response of many neurons in the brain to a tone or a flash of light, the response would not only reveal the nature of the event, but the other events that preceded it and when they occurred.

    The UCLA team tested the model by asking research volunteers in the study to judge the interval between two auditory tones under a variety of different conditions. The researchers found that volunteers’ sense of timing was impaired when the interval was randomly preceded by a “distracter” tone.

    “Our results suggest that the timing mechanisms that underlie our ability to recognize speech and enjoy music are distributed throughout the brain, and do not resemble the conventional clocks we wear on our wrists,” said Buonomano.

    Because time-related information is critical to understanding speech, determining how the brain tells time represents an important step toward understanding the causes of diseases, such as dyslexia, that result in impaired linguistic abilities, he noted.

    The next step for the research will be recording the response from a large number of brain cells to determine whether they encode information about the timing of stimuli.

    Buonomano collaborated with Uma Karmarkar, now a postdoctoral fellow at the University of California, Berkeley. The National Institute of Mental Health funded the study.

    Note: This story has been adapted from a news release issued by University of California – Los Angeles.

  7. #7 coturnix
    February 3, 2007

    Why did you just copy and paste a ScienceDaily report on a completely different topic – one I already posted asbout in another post?

  8. #8 Jonathan Vos Post
    February 3, 2007

    Sorry, coturnix. I didn’t see your prior posting. I’d cut & pasted the press release, and then didn’t quite know where to put it.

    I thought the bunker studies were deep cave studies, with no environmental periodicity of light, temperature, noise, or other variables. People decoupled from circadian rhythms, always going to longer days which they thought were still celestial days, even when roughly 30 to 40 hours in length. Poorly remembered anecdotal. I have a professor friend whose circadian rhythm seems broken. He does works for 3 days or more straight, fueled by coca cola, then crashes. Been doing that for many years. Perhaps also side effects of pain medication and tenure fight stress.

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