My latest research was published in Journal of Biological Rhythms this week. It is open access here: Casein Kinase 1 Underlies Temperature Compensation of Circadian Rhythms in Human Red Blood Cells.
Air travel challenges our bodies in a way that has never before been encountered in our evolutionary history. It allows us to move rapidly across multiple timezones, quicker than we could have ever moved by foot or animal. Unfortunately, our bodies are unable to adjust quickly enough. We are constrained by our circadian clocks, the things that give our bodies a sense of internal time, which have evolved to coordinate our physiology to the rhythmic and predictable changes in the external environment (as well as other roles) like day and night. The clock keeps its original time when you move timezones like a watch before you’ve reset it. It’s resistant to rapid change, giving us jet lag. However, unlike a watch, the body clock can gradually reset itself over a period of days so that we become tuned to a new local time. Despite this inbuilt mechanism, in an era of global travel it is often too slow.
Is it possible to speed up the resetting process? Can we travel the world without jetlag?
To many people, the phenomenon known scientifically as the circadian rhythm is bleeding obvious. We sleep in the night and are awake during the day, long-haul flights like those from the UK to Australia gives you jetlag, and night shifts are a right pain in the bum. Detailed explanations involving transcription-translation feedback loops and phase response curves don’t change those facts, they’re a fact of life when we live on a rotating world. But many scientists, myself included, are fascinated in the details, and some scientists, like Céline Vetter and colleagues at the Institute of Medical Psychology at Ludwig-Maximilian-University in Munich, use this eye for detail to find out how we might best cope with our biological timing in a 24-hour society.
Almost every animal and plant on the planet has a circadian clock, even those that live in the depths of the sea and deep underground in caves.
The presence of clocks in almost all life-forms implies that it is a helpful or advantageous characteristic, an evolutionary adaptation, serving to improve the fitness of the organism. This argument makes apparent sense but, without testable hypotheses, has little to support it.
Two main hypotheses have been formed to explain the evolutionary benefit of having a circadian clock. The first is known as the External Synchronisation hypothesis – that the benefit to the circadian clock lies in being coordinated with the external environment, for example, the predictable daily change in light and dark that we call day and night. The second is the Internal Synchronisation hypothesis – here the clock benefits an organism by allowing it organise physiological processes in time in order to avoid conflict between incompatible processes, for example separating the process of photosynthesis from that of nitrogen fixation in the case heterocystous cyanobacteria.
These two hypotheses aren’t mutually exclusive. The internal synchronisation hypothesis doesn’t necessarily require a 24 hour clock; plenty of other periods would suit. But, timing pressure placed on an animal from the external environment could force biological processes to fit within the 24 hour day, for example, the reactions for photosynthesis. These are only necessary in the day when it is light. But since nitrogen fixation and photosynthesis are incompatible, nitrogen fixation gets restricted to the night and internal organisation has been forced on an animal from external pressure. Once established, internal synchronisation could become independent of external pressures. Perhaps, in the origin of circadian timing systems, external synchronisation came first and internal synchronisation second, but now, either one serves as a selective advantage. So, though the two hypotheses propose reasons for selective advantage a circadian clock might give to an organism, and therefore why Clocks may have evolved in the first place, arguments are complicated as whether these are the original selective pressure that formed a circadian timing system.
How might we test which which hypothesis is most important today? One way is to look at animals that live in non-rhythmic environments, those that do not experience the regular and predictable cycle of day and night. These animals offer the chance to directly test the first, external synchrony, hypothesis, since in a non-rhythmic environment, there is no need to synchronise to an absent cycle.
The deep sea and caves are two environments that fit this description. Interestingly, most studies on organisms that live there give at least some hints that circadian clocks are present and working even here. Although, there are many difficulties interpreting and comparing this research due to the various experimental conditions used, this general observation lends weight to the internal synchronisation hypothesis – in the absence of a no cycling external environment, a ticking clock must be being used for something else, and internal synchrony is the most obvious.
My PhD research looked at one organism that lives in the depths of caves and is highly adapted to life there: the Mexican blind cavefish, Astyanax mexicanus. This fish shows wonderful daily patterns of behaviour and gene expression, confirming that it has a functional circadian clock. It also shows some interesting quirks, which give some insight into why an animal that lives in the dark and has done so for tens to hundreds of thousands of years might keep a system that generates 24 hours rhythms in physiology and behaviour.
Science requires controlled and well-planned experiments. Without correct set-up, results from experiments may not be reliable enough to be trusted. Circadian biology is no different in that regard, and especially when trying to find out if something has a working circadian clock, controlled experiments are crucial. Continue reading “How do you study circadian rhythms?”