"ON CIRCADIAN RHYTHMS"
Adaptations to our environment are of two types: REACTIVE, in which the adaptation follows an environmental change, and PREDICTIVE, in which the organism anticipates the environmental change before it takes place. If we do not consider universal entropy (because universal time scale is much larger than evolutionary time-scale and our own lives are much smaller compared to evolutionary time-scale), the environment we live in is very organized - everything seems to follow a pattern (as has already been discussed in the introduction to this website) and have a general rhythmicity to it. Circadian rhythmicity (as you might have already guessed, as a smart Reedie that you are!), is a predictive adaptation to the external environment - especially to the most important zeitgeber, light (which in turn shows light-dark phases due to Earth's 24-hr rotation).
Circadian clocks are thought to enhance reproductive fitness because an organism with circadian periodicity can regulate its physiological/behavioral activities in synchrony with that of the 'umwelt' (an animal's subjective environment) around it and maximize its survival chances. However, before 1970's, most of the evidence that supported the adaptiveness of clocks was not rigorous and fell into the category of pseudo-scientific storytelling. In other words, it made intuitive sense that being tuned to one's environment would enhance one's survival chances (and hence, one's reproductive success). As a result, any evolutionary biologist can call circadian rhythmicity an "important biological phenomenon". It is only after 1970's that ample evidence have emerged supporting the hypthesis that the daily periodicity of behaviors and metabolic activity enhance fitness - for example, an endogenous mechanism allows the anticipation of regular daily events such as dawn or dusk.
It makes perfect sense to us to say that circadian rhythm is an "important biological phenomenon", because it is easy for us to think (or the most "parsimonius" way of thinking!) that if a biological phenomenon is present, it must have been selected by evolution and therefore of adaptive significance. Stephen J. Gould and Richard Lewontin in their influential paper entitled "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" (1979), called this notion "adaptationist program" that provided explanations of biological processes that are intuitively/intellectually satisfying but might have a little basis on the history of evolution. Many biological processes might have evolved:
(i) as a random trait that was neither adaptive nor nonadaptive,
(ii) as a trait physiologically lnked to another trait where the linkage is either not currently present or not obvious,
(iii) as a trait that was once adaptive, but is now vetigial (like human appendix),
(iv) as a trait that evolved for one purpose but later was recruited to another task.
For instance, one can consider noses and ears, and how they serve as the perfect mounting platforms for spectacles. In the absence of knowledge of history of noses, ears and spectacles, it would be easy to assume that noses and ears evolved to provide support for spectacles to reside.
Adaptive Significance of Clocks:
Extrinsic versus Intrinsic Value of Biological Time-Keeping-
The original adaptation of circadain clocks was presumably to enhance reproductive fitness in natural environments, which are cyclic (24 hour) conditions - in other words, it is an extrinsic adaptation. However, it is also arguable that circadian clocks may additionally provide an "intrinsic adaptive value, i.e., circadian pacemakers may have evolved to become an intrinsic part of internal temporal organization and , as such, may have become intertwined with other traits that influence reproductive fitness in addition to their original role for adaptation to environmental cycles. One might not consider a trait to have any adaptive value if it has lost its extrinsic value. If , however, the clocks retain external value and additionally accrue internal value, they would be considered an adaptation. If it is true that circadian clocks have acquired intrinsic value for internal temporal programming, they would be expected to be of adaptive value to an organism in constant environments where the original selective pressure has relaxed. To support this hypothesis, populations of Drosophila melanogaster raised for hundreds of generations in constant conditions retain rhythmicity and the ability to entrain to various LD (light-dark) cycles, indicating that even in the absence of envrionmental selection, the components of the circadian system are mantained. A counter example is that of cave animals that frequently lose robust behavioral rhythmicity in the constant environment of caverns.
The daily clock is crucial for longer term processes in many animals. Migration, hibernation, fattening, and fur growth are all adaptations to winter, while the annual rut of large animals and the summer population explosion of smaller ones are all cued, months in advance, by the change in day length. The circadian clock is central to this effect because the signal it gives out changes its shape to reflect the longer nights of winter (thus, a predictive adaptation). As a result, the nocturnal peak of melatonin secretion by the pineal gland, which is tightly controlled by the suprachiasmatic nuclei, provides an internal endocrine calendar. A lengthening melatonin signal from night to night indicates the season is moving through autumn to winter, while progressive shortening means the worst of winter may soon be over.
WHAT'S UP WITH HUMANS?
So the daily clock provides an endocrine calendar, but is it important for humans? There are certainly reports of seasonal changes (this would be circannual rhythm, which in turn is based on the revolution of Earth around the Sun in 365 1/4 days) in mood, especially winter depression with atypical features of increased appetite and improvement by bright artificial lighting, but for most of us, our physiology remains almost immune to season (or that we think we are!). However, recent studies have shown that the photoperiodic timing system (refer to Fig.2 to see why I call this "photoperiodic") may be latent in our bodies, especially for sleep. Human Crusoes marooned under a summer-like long day length have a single consolidated nocturnal sleep bout, a sharply defined core body temperature minimum, and a short melatonin signal. When the nights are prolonged, the melatonin profile lengthens, sleep breaks into two components at the beginning and end of the night, with an intervening interval of quiet wakefulness, and the nocturnal rock-bottom of the core body temperature rhythm either broadens or shifts phase towards one or other sleep interval. This vestigial seasonal reorganization of the circadian system is probably not of great importance to most of us living a clockwork modern life. However, the subjective descriptions of the "quiet wakefulness", with the mind hovering back and forth between dream-filled sleep and conscious awareness suggest a deep mental/psychologial resonance with season which may underlie seasonal changes in normal and disordered mood. Perhaps we do all have a primitive need, driven by our clock, to turn down the lights, put another log on the fire, and sit back and rest, waiting for the winter to pass.