There was a time when, at the dawn of its formation, the future field of chronobiology was perceived as pseudoscience — “biorhythmology,” akin to astrology, with its mystical flavor. But, almost before you knew it, the discovery of circadian clock genes in the 1980s was awarded the Nobel Prize (in 2017). A remarkable shift in scientific credibility within a few decades. Seven key clock genes were found to determine the heritable speed of individual circadian clocks which, among other effects, divides us into different chronotypes with different sleep times — the larks and owls, whose clocks run shorter or longer than 24 hours.
However, there were major earlier discoveries bridging these events. In the 1950s, the pioneers Colin Pittendrigh and Jürgen Aschoff identified the formal properties of circadian systems — oscillation speed, relationship to the light-dark cycle, phase shifting to light — which provided us with a systematic understanding of circadian behavior and tools to control it in the lab. Twenty years later, the neural basis for body rhythms was identified in a tiny region of the brain near the optic nerve, the suprachiasmatic nuclei (SCN). This provided key anatomical proof that circadian rhythmicity was an integral part of body physiology and behavior, with a localizable pacemaker or master clock in the central nervous system.
Another 20 years of research culminated in the breakthrough discovery of melanopsin-containing photoreceptors in the retina of the eyes. Their function was not to form visual images, like the rod and cone photoreceptors, but to mediate the effect of light on physiology and behavior controlled by the central nervous system. These include effects on circadian rhythms (via the SCN), hormones (melatonin, cortisol, reproductive hormones), the neurovegetative nervous system (heart rate, pupil size), and cognitive behavior. The eye was thus established as a dual-function organ — both image-forming and non-image-forming — mediating multiple actions of light on the body. This reminds us of the dual function of the ear, with its separate controls over hearing and vestibular balance.
Yet another leap in understanding, thanks to the work of chronobiologists, was the recent identification of the perihabenular nucleus, which transmits the effect of light on the eyes directly to mood-regulating areas in the brain cortex. This pathway may mediate the well-known mood-elevating, antidepressant effect of light — particularly as a first-line treatment for winter depression.
Thus, in the public eye, chronobiology has moved completely from a pseudoscience to one of the most intensive areas of scientific investigation in the last 40 years. The basic laboratory work is translating into momentous practical advances for human health, opening new fields of chronotherapeutics, chronohygiene, and even chronoarchitecture — the raison d’être of the Center for Environmental Therapeutics. I myself followed this path from a curiosity about “biorhythms” to becoming a biomedical researcher studying both clinical applications and basic mechanisms.
Konstantin V. Danilenko MD, Institute of Physiology and Basic Medicine, Novosibirsk, Russia, is a biomedical researcher focusing on light physiology in humans. He has conducted a series of basic and clinical studies on biological rhythms, melatonin, winter depression, and reproductive function. He has been a member of the Society for Light Treatment and Biological Rhythms since 1989.