Our natural daily rhythm from rise to bed time is regulated by two different mechanisms: The first is the endogenous so-called circadian or biological clock that regulates cell activity on molecular level to tune to the 24 hour rhythm of our rotating planet. The second mechanism is light-dark cycle of the sun, which entrains this molecular clock by light input. A miss-alignment of the circadian clock as caused by shift work and rotating shift work is found to be associated with diseases like obesity, diabetes and cancer. The rise of these diseases may in part be associated with the all-day availability of light, food and recreational activity in our way of life, further demonstrating the need to understand our biological clock.[1] An additional benefit would the possibility to adjust the circadian clock of totally blind people. It has recently been found that their circadian clock is ‘free-running’ with a day length above 24 hours. Therefore the patients drift further and further away from the real light-day cycle they are living in, resulting in a row of health issues.[2]
To assess the influence of the light-dark cycle on the circadian clock arctic mammals are a conclusive choice, since they live in either complete darkness or complete light for most of the year. One model organism is the reindeer which has been found to lose daily rhythmic activity during the midnight sun.[3] In mammals the light-based entrainment of the circadian clock is carried out by light sensitive cells different from the neurons of the retina. Signals of these cells orchestrate production and secretion of the hormone melatonin, which is thought to act as an expression of sleep-need.[4] A recent study showed that melatonin production in reindeer also seems to be low and arrhythmic during arctic winter, suggesting an absence of circadian clock control. During the equinoxes, brief periods of ‘normal’ light-dark cycles at the poles, reindeer’s melatonin production seems to be solely controlled by light directly.[5]
So far it is only clear that the reindeer’s clock works significantly different from ours. It is not even clear whether a loss of time keeping is a general advantage for arctic mammals.[6] The valued reader may convince himself that the matter becomes even more complicated for cave fish dwelling in the dark[7] or for the arctic krill which acts as a timekeeper for a whole ecosystem by being its fundamental source of nutrient.[8]
Felix Spenkuch
References:
[1] M. D. Li, C. M. Li, Z. Wang, Yale J Biol Med 2012, 85, 387-401.
[2] R. L. Sack, R. W. Brandes, A. R. Kendall, A. J. Lewy, N Engl J Med 2000, 343, 1070-1077.
[3] B. E. van Oort, N. J. Tyler, M. P. Gerkema, L. Folkow, A. S. Blix, K. A. Stokkan, Nature 2005, 438, 1095-1096.
[4] V. Simonneaux, C. Ribelayga, Pharmacol Rev 2003, 55, 325-395.
[5] W. Lu, Q. J. Meng, N. J. Tyler, K. A. Stokkan, A. S. Loudon, Curr Biol 2010, 20, 533-537.
[6] M. J. Paul, W. J. Schwartz, Curr Biol 2010, 20, R280-282.
[7] N. Cavallari, E. Frigato, D. Vallone, N. Frohlich, J. F. Lopez-Olmeda, A. Foa, R. Berti, F. J. Sanchez-Vazquez, C. Bertolucci, N. S. Foulkes, PLoS Biol 2011, 9, e1001142.
[8] M. Teschke, S. Wendt, S. Kawaguchi, A. Kramer, B. Meyer, PLoS ONE 2011, 6, e26090.