*Geek Box: Circadian Rhythms
What are ‘circadian rhythms’? The term ‘circadian’ is derived from the Latin ‘circa’, meaning ‘around’, and ‘diem’, meaning ‘day’: ‘around the day’. Circadian rhythms provide an organisation to internal physiological processes, and are synchronised with our external environment. Circadian rhythms are considered an evolutionary advantage, as they allow an organism to anticipate changes in its environment. This allows an organism to anticipate and coordinate the appropriate responses relative to the time of day, for example desire to sleep in the evening, or appetite in the morning (in humans, of course…some animals are nocturnal!).
In humans, light is the most potent signal to synchronise our internal circadian rhythms with the external environment. The suprachiasmatic nucleus [SCN], located in the hypothalamus, is the master circadian control centre. In the eye, specialised cells contain a light receptor, known as melanopsin, which detects and responds to light, and relays these signals directly to the SCN through a dedicated pathway. These specialised cells, and the “master clock” in the SCN, are maximally sensitive to blue light with a short wavelength of 460-480 nanometres [nm]. This wavelength, which provides the blue colouring for the natural daytime sky, provides the primary daily signal for the master circadian clock by activating “CLOCK” [and “BMAL1”] genes in the SCN, which are responsible for generating the 24-hour circadian rhythms in cellular and endocrine processes.
However, although light is the primary entraining signal to the SCN, circadian rhythms in peripheral tissues – including the intestines, liver, heart, lung, adrenals, and adipose tissue – are synced through other external periodical time-cues, including meal timing and physical activity. Peripheral tissues have their own self-sustaining circadian rhythms, and also require input from the master clock to maintain rhythmicity. Their self-sustaining rhythms are also synchronised through meal timing and the cycle of feeding/fasting, which intertwines with the light/dark cycle. Meal timing is thus one of the most important external time-cues and synchronisers for peripheral tissues.