*Geek Box: Melatonin and the Circadian System
In humans [and all mammals], the suprachiasmatic nucleus [SCN], located in the hypothalamus, is the source from which circadian rhythms are generated. “Circadian” means “around the day”, and circadian rhythms are rhythms in our biological processes that are generated in the body and can synchronise to the 24 h day. We require circadian rhythms to synchronise to the light-dark cycle in order to appropriately time when, for example, we have a drive to sleep and drive to wake up, and for when the processes involved in metabolism of food are optimal.
Melatonin is produced and secreted from the pineal gland, but the rhythm in melatonin is generated by the SCN. The precise timing of the melatonin rhythm in a 24 h period is highly sensitive to time cues from light, specifically the absence of short wavelength blue light [picture the colour of the sky], and low colour intensities, influence the timing of melatonin release.
As a result, the melatonin rhythm is maintained at basal, near undetectable levels during the light period of the day, before rising and peaking during the biological night or dark phase. However, with modern environments creating artificial light exposure at night, and with humans in industrialised countries spending a majority of the light period in enclosed buildings, these factors can influence the timing and strength of the melatonin rhythm.
Melatonin is crucial to the overall regulation of the circadian system and our ability to regulate to the 24 h day/night cycle, however, the metabolic interactions and effects of melatonin are starting to be better understood, and could provide an explanation for the associations between food intake during the night and metabolic disease risk.