We’re all aware of our natural body clock pattern: some people are early birds, some people are night owls, a phenomenon known as your chronotype. You can override this with alarm clocks and coffee, which is especially important for shift workers. But have you ever noticed your chronotype shift when you go on holiday, especially when you holiday in the great outdoors?
Researchers from the University of Colorado wondered exactly that and described in 2013 interesting findings on the influence of natural and artificial light on human circadian clocks.
To do this they took 8 healthy people and put them in two environments. The first was one week in an artificial ‘everyday’ environment set up like their home lives, maintaining their daily routines of work, school and social activities and having electric lighting; the second was one week of camping outdoors with only natural light, i.e. no electric light from torches or phones, just light from the sun, campfires and the moon and stars.
They took three types of measurements in these conditions: amount of light exposure, activity levels and sleep timing, and the production of a hormone, melatonin, a biomarker of the internal circadian clock, present in their saliva. Melatonin is secreted in a tightly circadian manner where the start of secretion and the end of secretion represent the beginning and end of the internal biological night, respectively. They also determined the subjects’ chronotypes through two questionnaires – the Morningness-Eveningness Questionnaire and the Munich Chronotype Questionnaire – which ask questions regarding your normal sleep and activity patterns.
They show that “electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14 hr 40 min:9 hr 20 min light-dark cycle camping.” In other words, when in a natural light condition you get more bright light during the day, less light after dark (obviously), and the result is that your circadian clock will be shifted to earlier in the day.
These results help us understand a paradox in neurobiology – that in everyday conditions, the circadian low point of brain arousal, defined by cognitive performance and physiological markers of sleepiness like melatonin level, occurs 2 hours after most people wake up. This means that, from a circadian perspective, we are most sleepy after waking, not while we are still asleep.
The results here confirm this effect: in everyday conditions, melatonin offset, the marker for the end of the internal biological night, occurs about 3 hours after sunrise and about 1 hour after the end of sleep in the everyday environment. However, in the natural conditions, the melatonin turns off about 2 hours earlier than ‘everyday’ conditions, and about 50 minutes before wake time. This shift earlier would mean that the low point of brain arousal would be when we are still asleep, which makes more sense. Wright and colleagues’ results therefore suggest that the paradox may be simply be a consequence of the effects of our indoor lifestyle and electrical lighting – which delay our internal clock such that it doesn’t match sleep patterns.
Another interesting result to come out of this study is related to people’s individual chronotypes. The clocks of all participants were shifted earlier in the natural conditions relative to the ‘everyday’ conditions, but the amount the circadian clock shifted was larger for people with later chronotypes. In other words, the internal clock of those who were night owls shifted more than the clock of the early birds in the natural conditions. Looking at this the other way round, this suggests that the increased light exposure after sunset in everyday conditions exacerbates night owls’ tendencies to stay up – their clocks are forced later.
This second result has really become clear to me since moving to Mozambique. I’m a night owl, not strongly, but I’m not really a morning person and I find it easier to stay up late than get up earlier. But in Mozambique, being more like the experimental conditions of the natural environment rather than the ‘everyday’ environment from the paper (for example very bright sunlight and sometimes dodgy electricity at night), I am getting up a lot closer to sunrise than I would in the UK. I spend more time outside, meaning that I am exposed to more of the very strong entrainment signal that is sunlight, and so I bet my melatonin profile would look more like that during camping than the ‘everyday’ condition. My clock is shifted earlier by this more outdoors lifestyle.
How could I test that here? Without a lab to measure my melatonin production, I would have to rely on other measures of my internal clock to get an idea of my internal clock timing. Body temperature, blood pressure, reaction time, muscle strength, and bowel movements all show circadian variation, but since I would want to measure during sleep as well as wake, I’d have to choose an automated process which can be done whilst asleep. That rules out measuring reaction time, muscle strength and bowel movement timing (thankfully). It is possible to measure blood pressure automatically whilst you sleep, but you need equipment that isn’t easy to get hold of here, so that’s also out. My best option here is to measure core body temperature, a possibility with relatively cheap equipment.
Combining this with a measure of chronotype (for example the Munich Chronotype Questionnaire, I could get an idea of whether and how I’ve shifted since coming to Mozambique. I’d have to fill in the questionnaire as I was when in a previous job in the UK to see what the effect of Mozambique is on my ‘habitual’ or normal state, like Wright and colleagues do in this study. Ideally I’d also have a temperature profile measurement for my ‘habitual’ UK state but even without it, by comparing the chronotype and temperature profile I could get an idea of whether I have shifted. Perhaps for more data points I could also measure some friends to see if they have changed since moving here. It would be an interesting little experiment!
Have you ever noticed a change in your sleep pattern when you are doing outdoorsy activities? Do you agree with what they found in this study? I’d love to hear your thoughts.
Wright et al., 2013, Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle, Current Biology – author manuscript available from Pubmed here