Can't Switch Off

Is artificial light disturbing our circadian rhythms and ruining our sleep? As Andrew Beale explains, the answer is not quite that simple

The Biologist 64(6) p22-25

Do you get enough sleep? Recent documentaries on television – Sleepless Britain (BBC One, 6th March) and The Truth About Sleep (BBC One, 11th May) – painted a picture of a country suffering the effects of chronic sleep deprivation linked to our modern lifestyles.

These effects go beyond feeling drowsy at work: long-term reduction in sleep has been linked to the increased incidence of non-communicable diseases such as obesity, heart disease and diabetes[1].

While there is a healthy debate about whether modern society is experiencing a sleep loss epidemic, with a decrease in average sleep duration, this is a really difficult question to study. You would, ideally, perform objective measures of sleep timing in the same individuals over time. Asking successive generations to estimate how much sleep they get on average is much more imprecise. With this limitation, we have no consistent proof that average sleep duration has decreased over recent decades.

We do, however, have some firm examples that are concerning. Trends have been observed in the increased prevalence of city dwellers sleeping less than six hours a night[2]. There has also been a significant increase over the last couple of decades in the proportion of adolescents who get less than seven hours of sleep[3,4]. These changes in sleep patterns have been blamed on many factors, including changes in working habits, stress and diet, but perhaps the factor that is blamed most often is artificial light at night.

Eroding away the night

The hours of darkness, in which the diurnal Homo sapiens consolidates its sleep, are continually eaten away by our use of artificial light. Lights enable people to perform activities that are not possible in darkness and extend the useful day beyond the restrictions set by sunrise and sunset.

Together with the increased use of electronic devices, our eyes are being exposed to light during more hours than in previous decades. This extra light, from street lighting, household lighting, computers and smartphones, directly affects our physiology.

Light is the principal external signal that regulates the human internal timing system known as the circadian clock (see 'Circadian rhythms', below). Controlled by the suprachiasmatic nucleus (SCN) in the brain, the circadian clock gets information from the retina about the external light-dark cycle, ensuring genetic and neuronal oscillations are set to local time. The SCN coordinates the timing of cellular clocks in every cell of the body, keeping our physiology in sync with the outside world. Jet lag is an effect of desynchronisation between the internal timing of the body and environmental conditions.

Through changes to our built environments, we can enforce similar disruption to our body clocks without trans-continental flights – most obviously through shift work. When light is perceived in the evening, the fine-tuned coordination (or 'entrainment'), is disturbed, delaying the circadian clock and the processes it controls away from local time. These include secretion of the night-time hormone melatonin, and also sleep.

Sleep diagramCircadian Rhythms - Humans, like other organisms, show circadian rhythms in behaviour, metabolism and physiology, including regular sleep and wake cycles. These rhythms originate from a mechanism found in each cell of the body that is made up of complex transcription-translation feedback loops involving specific clock genes, as well as post-translational modification, which generate near 24-hour rhythms in cell function. Almost every cell contains this timing system, but the rhythms need to be coordinated and synchronised to the external environment. In mammals, this is driven by a region of the hypothalamus of the brain known as the suprachiasmatic nucleus (SCN). The SCN acts as a master pacemaker and coordinates timing via the control of body temperature and the secretion of hormones such as melatonin into the blood stream. When animals have the SCN removed, or key clock genes disabled, they become arrhythmic in the absence of a light-dark cycle, meaning that they do not show regular and predictable oscillations over a 24-hour period.

So, light at night can put pressure on our sleep by delaying our clocks and our bedtimes. But is Thomas Edison, inventor of the light bulb, (who famously considered sleep a waste of time) to blame for our bad sleep habits?

Perhaps, but there are other aspects to consider. One is the constraint on our sleep patterns arising from the standard '9 to 5' schedule of work or school in modern, urbanised society. We use alarm clocks to comply with these social demands, which often prematurely and abruptly terminate our sleep.

When these demands are fixed, and yet we stay awake late at night, sleep duration decreases. If our lives were not constrained in this way, later bedtimes might be compensated for by an extension of sleep in the morning. Instead, we are socially jet-lagged, meaning that our bodies are out of sync with our social schedules and we typically make up for working week habits by sleeping in at the weekend.

Another aspect is our indoor lifestyles. The circadian clock only reacts to external (environmental) light at two fairly narrow time windows in the day. Light exposure in the late evening will delay the circadian clock, while light during the morning hours has the opposite effect and will advance it.

Unlike our ancestors, most of us spend most of our time inside buildings, reducing our exposure to bright day-time light by an order of magnitude[5,6]. There are two particular problems with this. First, we get less light indoors in the critical time window of the morning, which we need to advance our clocks. Second, we expose ourselves to delaying light in the evening, when our ancestors had none. Together, this leads to later circadian clock timings and later sleep schedules in modern society.

However, there is a great deal besides electricity and light exposure that is associated with modern life that can also affect sleep – such as diet, stress, alcohol, consumption of stimulants, and the 'always on' nature of smartphones and social media. Since whatever changes that have caused our modern sleep patterns have already occurred, it is important and helpful to study areas of the world where this transformation is still an ongoing process. Continuing urbanisation in many areas of the world affords us this opportunity.

Naturalistic sleep and urbanisation

'Natural sleep' exists no more than the natural habitat in which our species evolved, which was different from the houses we have built around ourselves. Clearly, electrification was not the first change we made to our environment that affected our sleep. It came with industrialisation, which created the imperative of being at work during certain hours of the day. So sleep researchers have become increasingly interested in studying societies in which industrialisation has not yet happened or is on the cusp of happening. A number of studies have been conducted in areas with varying degrees of urbanisation with and without access to electricity.

One method that has proven extremely useful in assessing and characterising sleep in developing countries and pre-industrial societies is wristwatch actimetry, using devices such as a Fitbit to measure physical activity. Without interfering with the daily activities of the volunteers or requiring any action on their part, actimeters record activity and rest, as well as light exposure, for several weeks. Movement data helps calculate when the individual was awake and asleep, as well as disturbances during their sleep.

One study by a team of sleep researchers and anthropologists[7] was performed in three hunter-gatherer communities living in rural parts of Africa and South America. It found that sleep duration in the three communities was at the lower range of those reported in post-industrial societies, between 5.7 and 7.1 hours, suggesting that our expectations of eight hours sleep per night may not be based on what we were biologically adapted for.

However, this interpretation is controversial, partly because of concerns that the sleep patterns observed in these hunter-gatherer communities are based on the need for survival rather than preference or general wellbeing, and may also be unique to the specific context of these communities. Hence, there is disagreement as to how broadly applicable these results are to our general understanding of human sleep and how human sleep may have evolved.
A useful model is to investigate sleep in populations in areas with varying degrees of urbanisation, including neighbouring populations with and without access to electricity.

Studies on such communities in the Amazonian region of Brazil, the Chaco of Argentina and northern Mozambique have consistently shown that sleep and circadian clock timing is delayed in an environment with access to electricity, even when that access may consist of a single, dim, electric bulb in a home.

These results, from a natural context, agree with data from studies in which people are taken out of a post-industrial environment. Ken Wright at the University of Colorado and his colleagues, for example, examined the internal circadian timing and sleep schedules of a group of subjects before and during a camping trip in the wilderness[6]. When in their 'normal' electrified environment, the participants were exposed to more light after sunset and less during the day compared with when they were brought out into 'natural' conditions (no torches or electronic devices, sleeping in tents). The non-electrified conditions advanced their body clocks by two hours. These studies confirm that artificial light after sunset can delay the circadian clock and that exposure to bright natural daylight in the morning results in earlier rhythms and sleep patterns.

But what happens to sleep duration when a community gets electricity? Reports give different answers to this question. Studies in Argentina[8] and Brazil[9] show a significant reduction in sleep duration in communities with electricity relative to their neighbours who were culturally and ethnically similar, but lacked access to electricity. By contrast, work we have just published shows that sleep duration did not significantly differ between electrified urban and unelectrified rural communities of Mozambique[10].
Why was our finding different? We found that the Mozambican community that had access to electricity (most of them were self-employed small traders) fell asleep later, but also made up for this with a delayed wake time. In other words, by being able to and choosing to sleep longer, they avoid shortening their sleep.

By contrast, the Amazonian rubber tappers in São Paulo studied by Claudia Moreno and her team9 were required to work to strict schedules and so had to follow set wake-up times, like most of us in the UK and other post-industrial societies. Taken together, these observations seem to suggest that the delay in sleep timing associated with electrification is quite a separate phenomenon from acquiring a daily habit of partial sleep deprivation, which is highly influenced by societal pressures.

There is more to sleep than sleep duration, however. Quality of sleep is important for how refreshed we feel and how well we can perform the day after. Sleep quality can be negatively affected by external disturbances, or mental or physical factors, most prominently sleep apnoea, where our upper airwaves are closed during sleep.

Actigraphy can tell us how restless the sleep period is or how many times during a night a person wakes up. And actigraphy-based studies in rural and urban environments suggest that quality of sleep is closely related to comfort.

Modern city dwellers may think of the rural environment as an idyll of heavenly sleep, but in reality many people who live in such environments throughout the world do not have a comfortable bed, share their bedroom with other people and even animals, and may be aching due to heavy physical labour during the day[10–12]. Thus, from a global perspective, industrialisation and urbanisation is associated with generally greater sleeping comfort and higher sleep quality[10], 12

Artificial lighting is not alone, then, but forms part of a complex set of developments during urbanisation that are disrupting our sleep. On the one hand, these developments allow us to stay up longer and tempt us to do so through a variety of technological inventions. On the other hand, they also help us sleep better by giving us housing that keeps us safe and reduces external noise, with comfortable beds, thick curtains and the ability to control our indoor temperature.

It is not a given that we must pay for all these improvements by sleeping less. By understanding more about our biology, and how moving indoors and into cities has changed it, we may be able to strike a better balance than many of us manage to do today.

Evening light only precipitates into sleep loss when combined with early rising due to social pressures, and perhaps if bright light exposure during the day and particularly in the morning were encouraged, artificial light would not result in sleep loss at all.

That may require innovative design of living and working spaces to facilitate maximum exposure to light. However, before the rapidly urbanising world adopts our lifestyle and sleep patterns, we would be well advised to study and learn from theirs.

Dr Andrew Beale MRSB is a research fellow in the chronobiology section of the School of Biosciences and Medicine at the University of Surrey.

References
1) Rangaraj, V. R. & Knutson, K. L. Association between sleep deficiency and cardiometabolic disease: implications for health disparities. Sleep Medicine 18, 19–35 (2016).
2) Knutson, K. L. et al. Trends in the prevalence of short sleepers in the USA: 1975–2006. Sleep 33(1), 37–45 (2010).
3) Keyes, K. M. et al. The great sleep recession: changes in sleep duration among US adolescents, 1991–2012. Pediatrics 135(3), 460–468 (2015).
4) Kronholm, E. et al. Trends in self-reported sleep problems, tiredness and related school performance among Finnish adolescents from 1984 to 2011. J. Sleep Res. 24(1), 3–10 (2014).
5) Thorne, H. C. et al. Daily and seasonal variation in the spectral composition of light exposure in humans. Chronobiol. Int. 26(5), 854–866 (2009).
6) Wright, K. P. Jr. et al. Entrainment of the human circadian clock to the natural light-dark cycle. Curr. Biol. 23(16), 1554–1558 (2013).
7) Yetish, G. et al. Natural sleep and its seasonal variations in three pre-industrial societies. Curr. Biol. 25(21), 2862–2868 (2015).
8) de la Iglesia H. O. et al. Access to electric light is associated with shorter sleep duration in a traditionally hunter-gatherer community. J. Biol. Rhythms 30(4), 342–350 (2015).
9) Moreno, C. R. et al. Sleep patterns in Amazon rubber tappers with and without electric light at home. Sci. Rep. 5, 14074 (2015).
10) Beale, A. D. et al. Comparison between an African town and a neighbouring village shows delayed, but not decreased, sleep during the early stages of urbanisation. Sci. Rep. (UK) (2017, in press).
11) Martins, A. J. et al. Effects of physical activity at work and life-style on sleep in workers from an Amazonian Extractivist Reserve. Sleep Sci. 9, 289–294 (2016).
12) Samson, D. R. et al. Segmented sleep in a nonelectric, small-scale agricultural society in Madagascar. Am. J. Hum. Biol 29(4) (2017).

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