Twilight Times: Light and the Circadian System

Abstract

Whose twilights were more clear, than our mid-day.John Donne (1571-1631) Of the progress of the soul THE CIRCADIAN CLOCKThe spatial and temporal features of the environment have provided the abiotic selection pressures that have shaped the evolution of life on earth. We are accustomed to accept that the spatial world offers specialized niches. However, we rarely consider that the temporal structure of our planet offers similar opportunities. Most organisms have evolved specializations that allow them to exploit their environment in terms of both space and time, and this demands that individuals have an endogenous representation of these environmental features. We know surprisingly little about the endogenous representation of space, but in recent years an understanding of how organisms build a representation of their temporal world has begun to emerge.Organisms do not merely respond to their environment, they also have the capacity to adjust physiology and behavior in anticipation of changing environmental conditions. In a competitive world, "being prepared" offers a great selective advantage. It takes considerable time to bring about the complex realignments of physiological systems that permit an optimal expression of different behavioral states (e.g. activity and rest or exploitation of temporally restricted resources). By fine-tuning physiology in advance of the changing conditions, an organism can be ready to exploit the changed conditions to its best advantage.At the heart of the biological machinery that "creates" a day within us is a biological or Circadian clock. In mammals, for example, this resides within a small paired nucleus in the brain located above the crossing of the optic nerves, the suprachiasmatic nuclei or SCN.? The neuronal activity of this nucleus continues to oscillate with a 24 h rhythmicity

Circadian regulation of olfaction and an evolutionarily conserved, nontranscriptional marker in Caenorhabditis elegans

“…As a starting point, we note that although the strongest zeitgeber for the circadian clock is usually light (13), likely due to its predictability from day to day and year to year, C. elegans is soildwelling and not generally exposed to light. Organisms, like the nematode, that evolved within spatial niches devoid of light often lack energetically costly and complex light-shielding or -detecting mechanisms such as pigmentation or eyes (14).…”

Myopia, Light and Circadian Rhythms

“…The underlying circadian rhythm is controlled by a circadian clock, through the expression of "clock genes" and their products, and entrainment cues known as zeitgebers ("time givers") which influence the timing of these rhythms (Roenneberg and Foster 1997;Iuvone, Tosini et al 2005). The primary, central time keeping clock in the body is the suprachiasmatic nucleus (SCN) of the hypothalamus (Roenneberg and Foster 1997).…”

“…The presumed role of circadian rhythms, which persist in constant darkness independent of the diurnal light:dark cycle, is to regulate the timing of biological events so as to optimise the metabolic function and energy use of the organism (Roenneberg and Foster 1997;Levi and Schibler 2007).…”

Synchronization of theDrosophilaCircadian Clock by Temperature Cycles

“…The final degree of complexity is very likely still ahead of us, given that studies have so far been mainly conducted under strict 12-hour light:12-hour dark conditions, ignoring the changes of light quality and quantity during twilight. This is somewhat ironic, because light conditions during twilight represent a particularly potent zeitgeber (Roenneberg and Foster 1997). It therefore seems time to switch to more natural entrainment conditions in the lab (Bachleitner et al 2007) or even for conducting studies "in the wild.…”