A Sleep Discovery
Sleep loss problem? We often take for granted and even curse our bodies' daily incessant demand for sleep. That tide of tiredness we feel every night acts as a seemingly crude trigger for what is a complex coma-like state we fall into. How we control the timing and consistency of our sleeping patterns is precise and adaptive. Two processes in our body, our homeostatic drive and our circadian rhythm act together pushing and pulling our body into and out of its sleep state. Both of these physiological processes use different inputs and produce varying effects in a combined effort to force the body to sleep when needed.
One Third of Our Life
It is strange to think that we evolved to spend ⅓ of our living life in a defenceless state during which we can not protect ourselves, eat or reproduce. On the surface, it seems to go against a fundamental understanding of evolution - survival of the fittest. With evolution in mind, it would make sense that we would evolve to sleep less. If we look around at nature we begin to see other examples of this sleep state in action and the adaptations to facilitate it. Birds perched on a wire will take turns to sleep, with the birds at the end of the line remaining awake. While when a dolphin engages in sleep it will only switch off one hemisphere of its brain.
We are only beginning to scratch the surface in our understanding of sleep, sleep loss and sleep insufficiency from observing how intrinsic it is to our life and wellbeing even with its seemingly evolutionary contradictions. It is those contradictions and the marvels of evolution that help us begin to grasp its importance to our development and longevity.
The 2017 Nobel prize was awarded to the scientists who discovered the molecular mechanisms controlling our bodies circadian rhythm which is one half of the duo of physiological processes that control our sleep-wake cycles. They uncovered the self-regulating ability of the circadian clock in each cell and how the circadian rhythm in each cell is adjusted by the body's central circadian rhythm. One method the body used to adjust the timing of the cells circadian rhythm was through the modulation of the so-called ‘sleep hormone’ melatonin. Melatonin is almost exclusively produced in the pineal gland. The pineal gland modulates its release into the bloodstream in response to environmental light triggers from the retina. The daily modulation of melatonin levels informs our circadian rhythms of environmental light which act as a proxy for the rising and setting sun. Allowing our bodies to synchronise with the 24hr rotation of the earth ensuring optimum sleep and avoiding sleep loss.
For centuries this process of adjusting the timing of our circadian rhythms based on environmental light conditions worked great. The sun rose and set each day and that allowed our bodies to follow a similar pattern. However, in the early 1900s the widespread adoption of a new affordable light source, the incandescent light bulb began. All of a sudden we no longer depended on the sun quite so much.
Fast forward 100 years and most of us live in urban environments which are under artificial light 24/7 and sleep has become a nuisance. A lot of us work for 8 hours a day in front of a computer which blasts light into our eyes and in addition, we spend a further 4 hours a day staring at portable light in the form of a mobile phone. How can our body discern day from night using its physiological systems when it is been stimulated by artificial light sources from before sunrise and well into the night after sunset.
The CDC carried out surveys in 2005-2007 and reported that over ⅓ of adults in America sleep less than 7 hours in a 24 hr period.
Sleep Loss and Insufficient Sleep
In the United States the CDC, a federal agency in charge of public health, operates an ongoing public health study called the Behavioural Risk Factor Surveillance System. Through this, the CDC surveys U.S. residents regarding their health-related risk behaviours, chronic health conditions, and use of preventive services. Carrying out 400,000+ telephone interviews each year.
In 2008 a question was added to the core questions in the survey related to sleep loss and on review of the responses the CDC found that 11.1% of U.S. adults reported insufficient sleep or rest for all 30 of the preceding 30 days with 29% of U.S. adults report sleeping <7 hours per night. In 2011, after an analysis of the 2009 responses, the CDC reported that of the 74,571 adult respondents in 12 states, 35.3% reported <7 hours of sleep during a typical 24-hour period and 4.7% reported nodding off or falling asleep while driving at least once in the preceding month. From this analysis, the CDC classified insufficient sleep as a public health epidemic.
However, in 2015 the CDC changed their mind and re-classified insufficient sleep or sleep loss as a public health problem. They did not cite any new data from the Behavioural Risk Factor Surveillance System or other data that may have led to this change so we are left wondering why the change of heart. Giving that the National Sleep Foundation recommends 7-9 hrs of sleep per night for adults and that ⅓ of Americans reported sleeping less than 7 hrs per night you would not expect the CDC to degrade the severity of the issue.
If we look at another survey study published in 2013 which aimed to investigate whether the prevalences of short and long sleep durations have increased from the 1970s to the 2000s. In this study, the researchers analysed data from repeated cross-sectional surveys of 10 industrialised countries. Over the periods covered by the data, the prevalence of short sleep (proxy for sleep loss) duration was found to have increased in Italy and Norway but decreased in Sweden, the United Kingdom, and the United States. These results may go some way to possibly explain the CDCs decision to somewhat downgrade the problem around insufficient sleep levels. But considering the time period of the US data used in the study, 1985-2007, which is clearly dated before the 2008 and 2009 CDC Behavioural Risk Factor Surveillance System surveys we do not have a complete explanation.
If we take a step back, look at sleep patterns from a cultural perspective. Mediterranean cultures of which are found in Italy have traditionally followed a biphasic sleep pattern. A daily siesta was ingrained in their culture from the old days when manual labour was more common and a midday break gave people a chance to escape the hot sun. The 2013 study which covered 10 countries in total found there to be an increase in short sleeping and a decrease in long sleeping in Italy. This would indicate that the population in the country is sleeping less across the board.
This population trend is short sleeping might be explained by a cultural shift away from the traditional lifestyle of biphasic sleeping patterns being phased out by the modern 9-5 working practice. Removing the Italians opportunity for daytime naps leading to a total reduction in 24hr sleep time. Maybe there are cultural factors in the US which might explain the high levels of reported short sleep to the CDC and the contradicting decrease in the prevalence of short sleep uncovered by the 2013 study. It is difficult to say based on the available epidemiology data.
Ask the Right Question
When studying the sleep patterns at the population level it is difficult to gather accurate data at a large scale. The CDC’s Behavioural Risk Factor Surveillance System surveys large numbers of people but only asks recall style questions. The 2013 American Journal of Epidemiology study used Multinational Time Use Surveys which are better in terms of accuracy but can only be carried out at a smaller scale. A more recent 2018 study from Oxford University used similar time-use diaries to evaluate population level sleep patterns between 1974 and 2015 in the UK. That study found that people in the UK sleep today 43 min more than they did in the 1970s. Those results apply to men and women alike, and individuals of all ages and employment status, including employed individuals, the presumed major victims of the sleep deprivation epidemic and the 24/7 society.
The study goes on to conclude that sleep duration is not the best or the only parameter to describe the “sleep health” of a population. The quality of sleep should be considered as well, especially because recent studies found an increase in sleep complaints and links with increased all-cause mortality. With the recall style questions used in the CDC surveys the individual may recall reduced sleep duration when it was in fact a sleep quality issue.
So it appears the available data is unclear and conflicting. Changing demographics and cultural shifts are having an impact on traditional sleeping patterns along with the ever advancing technological development changing our lives at a rapid pace.
Moving away from sleep duration and looking at the connections between sleep and health. One of the primary reasons to ensure we achieve adequate quality sleep is to drive positive health and longevity. Regardless of our perceived sleep duration meeting prophesied optimal sleep duration levels we primarily want to maintain a healthy mind and body. So what about our body's dose-response to sleep? When should we be prescribing sleep as a treatment?
Let’s first look at an interesting phenomenon around daylight summer time saving. The largest double blind placebo study ever carried out which is repeated annually. After adjustment for trend and seasonal effects, it was found that the Monday following springtime changes associated with a 24% increase in daily acute myocardial infarction counts, and the Tuesday following autumn changes was conversely associated with a 21% reduction in the data studied.
No other weekdays in the weeks following daylight summer time saving changes demonstrated significant associations. Indicating that when we lose an hour of sleep there is an increase in presentations of individuals with heart attacks and when we gain an hour of sleep there is a reduction. The study notes that the impacts are likely only on the timing of presentations for acute myocardial infarction and not on the overall incidence of this disease. This presents an interesting starting point when looking at the dose-response to sleep.
Sleep As a Treatment
In 2016 there was a study published which carried out a dose-response meta-analysis to summarise evidence from prospective cohort studies about the association of nighttime sleep duration and 24-hour sleep duration with a risk of all-cause mortality among adults. Previous studies suggested that sleep duration, specifically short and long sleep durations, may be associated with the risk of all-cause mortality. This study aimed to analyse the risk of all-cause mortality associated with any specific duration of sleep.
Review of this dose-response for sleep leads to to a few potential conclusions:
directly causes mortality itself?
may result from variety of social, environmental, and physiological changes that lead to increased mortality risk?
itself causes physiological and social outcomes that may lead to increased mortality?
is associated with other characteristics causally linked to mortality, such as age?
long sleep is linked to increased sleep fragmentation that is associated with a number of negative health outcomes; long sleep is associated with feelings of fatigue and lethargy that may decrease resistance to stress and disease?
a lack of physiological challenge (sedentary lifestyle) with long sleep decrease longevity?
underlying disease processes mediate the relationship between long sleep and mortality?
the possibility of reverse causality is also of concern?
Drawing concrete conclusions on population level sleep loss or insufficiency is difficult and given that most of the studies are epidemiology in nature it is unwise. We could look at the U-shaped dose-response curve and conclude that 7 hrs sleep per night is an optimal duration target.
Sleep and Cancer
If we look at a specific disease like cancer instead of all-cause mortality we don’t get any more of an insight. While the International Agency for Research on Cancer has classified night shift work as a probable carcinogen as recently as 2019 while another systematic review of sixty-five studies from 2018 which involved 1,550,524 participants and 86,201 cancer cases found that neither short nor long sleep duration was associated with increased cancer risk.
Conflicting views but again we cannot over rely on epidemiology studies to infer causation.
Where does that leave us in our journey to understand our modern sleep epidemic? Conflicting research points us in different directions but there is a consensus at the extremes. Short sleep of fewer than 6 hrs and long sleep of over 9 hrs will mean you are at increased risk of serious health problems. With some countries populations apparently sleeping more and others not so much.
It is interesting to review the landscape on this topic from a high-level perspective and gain a holistic understanding of it. There have been changes in opinion over the last 10-20 yrs and by reviewing those decisions and the data that has led to them we can better understand where we stand currently. There are many more sub-plots to this story on sleep, health and longevity. This article is but a teaser on the high level concepts that have informed the zeitgeist today. In further articles, I will explore more niche topics on health and longevity as well as performance.
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