*Geek Box: Melatonin
Hearing terms like ‘circadian phase’ or ‘biological night’ can be confusing, particularly where we think of time usually by reference to the time on the clock. Both of these aforementioned terms refer to the 24hr circadian rhythm in the hormone melatonin.
The term ‘circadian phase’ reflects the fact that, if you were to go into a pitch black cave and divorce your senses from light or any other time cues that could indicate what time of day it was, melatonin would still follow a relatively similar rhythm over the day and elevations in melatonin would persist.
If, however, you flew across time zones and were in a new light-dark cycle, your melatonin rhythm would still be fluctuating according to the phase of your previous time zone. It would take a number of days for melatonin to align to your new light-dark cycle. Thus, measuring melatonin provides the most robust marker of an individual’s internal circadian phase, i.e., whether they are aligned with their local time and light-dark cycles.
The most robust measurement of melatonin is known as ‘dim-light melatonin onset’ [DLMO], and DLMO is measured by taking blood or saliva samples every hour overnight in dim/dark light conditions [usually 12 or 13 measures in total], and calculating the time at which 25% of peak melatonin levels are reached [as one method, it is also possible to pick a particular threshold over which plasma or saliva melatonin level rises as the marker of DLMO].
It is important to note that this is where individuals may differ in their circadian phase, i.e., their ‘chronotype’. Chronotype is a behavioural expression of time-of-day preferences, but those preferences reflect internal biological timing. If we took two individuals, one could have an earlier onset of DLMO at 20.00hr, while the other may have a DLMO at 01.00hr.
This is where the term ‘biological night’ comes in. For example, if both of these individuals were in a timezone where the sunset at 18.00hr, they would probably say that after 18.00 is ‘nighttime’ simply because it is dark outside. However, that is ‘clock time night’: the biological night for these individuals would start when melatonin rises, measured as DLMO. In this example, the ‘biological night’ for these individuals would differ in its timing because their respective DLMO timings differ substantially.
DLMO is attracting more interested in the context of the research on evening energy intake and bodyweight, as it may be that calorie intake in closer proximity to measured DLMO has more of a relationship with adiposity compared to only analysing calorie intake relative to clock time. Indeed, this is precisely what a number of recent analyses found (11,12): when analysing the relationship between evening calorie intake and body fat, the relationship was not evident when looking at clocktime, however, once calorie intake was analysed relative to DLMO – i.e., relative to individual internal ‘biological night’ – the association became significant. Thus, it may be that an individual’s circadian phase and biological timing of, among other circadian rhythms, their melatonin levels, is an important mediating factor in the relationship between timing of food intake and metabolic health.