*Geek Box: Melatonin, “Circadian Time” and “Clock Time”

*Geek Box: Melatonin, “Circadian Time” and “Clock Time”

Melatonin is produced and secreted from the pineal gland, but the rhythm in melatonin is generated by the suprachiasmatic nucleus [SCN] located in the hypothalamus. 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, which 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 the majority of the light period in enclosed buildings, these factors can influence the timing and strength of the melatonin rhythm.

It should be noted that each individual exhibits a different “chronotype”, i.e., a behavioural expression of their sleep-wake timing preferences, often colloquially termed “morning larks” or “night owls”. A chronotype may be determined by a measurement of the “dim-light melatonin onset” [DLMO]. In a night owl, DLMO occurs at a much later clock time compared to a morning lark, e.g., DLMO at 2 am in a night owl compared to 9 pm in a morning lark.

Thus, their DLMO would be a representation of what we would term “circadian timing”, as clearly the clock time at which DLMO occurs is very different in this example. Thus, when you see the term “clock time”, this means precisely that; the time as it would appear on your clock. However, when you see the term “circadian time”, this means a representation of an individual’s internal biological timing.