Newsletter #296: Sleep Timing & Mortality ⏰
In the realm of health, we put a ton of emphasis on increasing sleep duration. And rightfully so. It is well established that habitual sleep loss is linked to higher risk of death from any cause, as well as greater risk of developing an array of diseases.
However, emerging research suggests that when you sleep may be just about as crucial as the amount of sleep you get. Specifically, sleep regularity, meaning the day-to-day consistency of sleep-wake timing, has been found to be an even stronger predictor of certain health outcomes than sleep duration.
For example, irregular sleep timing has been found to be a significant risk factor for cardiovascular disease, independent of both sleep duration and traditional heart disease risk factors. In one observational study, people with the most irregular sleep patterns had more than double the risk of going on to experience a cardiovascular event, compared to people whose sleep timing was consistent.
A new study that collected objective sleep data from a large cohort found, remarkably, that sleep regularity was a stronger predictor of all-cause mortality during the follow-up period than sleep duration!
Let’s take a look at what the researchers did here, and what we should take away from it.
This Week’s Research Highlight
Sleep timing on a night-to-night basis is a stronger predictor of mortality risk than how long one sleeps.
A prior Japanese study that looked at the relationship between sleep parameters and risk of dying from any cause found that sleep irregularity increased risk of all-cause mortality by 30%, regardless of sleep duration. However, that study relied upon self-reported sleep metrics. And as it turns out, people aren't always so great at figuring out when or how long they were asleep, without the help of specialized instruments.
To examine how parameters of sleep predict mortality risk using objective sleep measurements, researchers analyzed data from 60977 participants in the UK Biobank Study. As part of the study, subjects had spent seven consecutive days wearing actigraphs — wrist-worn devices that can accurately detect when the wearer is asleep or awake, not unlike a Fitbit.
To assess sleep regularity, the researchers calculated a metric known as the Sleep Regularity Index (SRI). This metric determines the probability that someone would be asleep at any two time points that are exactly 24 hours apart; for example, 9pm on Friday and 9pm on Saturday. SRI operates on a scale of 0-100, wherein a SRI of 100 would mean that you are consistently asleep at precisely the same time on a day-to-day basis, and an SRI of 0 indicates that sleep timing is totally random.
Participants were followed for nearly eight years after the sleep recordings were completed to see how sleep duration and timing would later influence health outcomes, including mortality, which was captured from the UK national death register.
Participants with higher SRI scores (meaning between 71.6 to 98.5) had a 20-48% lower risk of all-cause mortality, compared to those with lower scores (meaning under 71.6). When they dug a little deeper into specific causes of mortality, they saw a 22%–57% reduced risk of cardiometabolic mortality and a 16%–39% decreased risk of death due to cancer in the group with higher sleep regularity, versus those with a lower score. Furthermore, this relationship remained after extensive adjustment to potential confounders, including exercise, smoking, age, pre-existing disease, BMI, lifestyle and socioeconomics, etc.
You might wonder: why exactly does it matter if you go to bed at around the same time on a day-to-day basis, as long as you are getting enough sleep in the first place?
We have evolved molecular clocks which align aspects of our physiology with the solar cycle. This helps prepare the body for changes in the environment that are associated with predictable behavioral patterns, like physical activity and eating.
When your sleep timing is unstable, your circadian system becomes disrupted, producing cellular alterations that eventually can contribute to disease. Just as one example, the activity of mitochondrial enzymes are regulated by circadian clocks, to ensure that you are burning more energy during the day, when you would normally be taking in and using energy. Circadian misalignment derails this diurnal rhythm, leading to impaired glucose tolerance, and ultimately to greater risk of going on to develop type 2 diabetes.
The good news is that you can do something about it. The main driver of circadian rhythms is light exposure, followed by some behavioral inputs like eating and exercise. Furthermore, a genome-wide association study looking at sleep traits estimated the heritability of sleep regularity at just 2.8% (sleep duration, in contrast, was 19.0% heritable). Taken together, this suggests that sleep regularity is highly actionable, because it does not appear to be strongly governed by genetics, and it is responsive to modifiable factors like environment and lifestyle.
So what should we shoot for here? Well, to put this into practical terms, most of the folks with the higher SRI scores would have been falling asleep (and waking up) within roughly one hour windows on a day-to-day basis, which I think is a fairly realistic target for most people.
Random Trivia & Weird News
There is a weirdly persistent myth out there suggesting that people accidentally consume, on average, eight spiders annually while they are asleep.
This statistic, however, is unfounded, and frankly implausible. It’s pretty unlikely that you have ever had a spider crawl into your mouth while you are asleep — let alone hundreds over your whole lifetime.
Don’t rely on the “real facts” printed under Snapple caps. Source: Matt Cohen, RD.com
Podcasts We Loved This Week
- Nicola Vannini: Revitalizing the immune system. Via The LLAMA Podcast.
- Sten Stray-Gundersen: The benefits of blood flow restriction training. Via STEM-Talk.
Products We Like
These are super cool. They’re smart bulbs that you can directly connect to Google Assistant or Alexa, and can dim or change colors at your command.
Obviously fun since you can make them whatever color you like, but also very useful from the standpoint of anchoring your circadian rhythms. During the day, you can make them bright blue, for maximal alertness, and then during the evening you can switch to more of a dim amber.
humanOS Catalog Feature of the Week
We evolved in the presence of natural daily cycles of light and darkness. But the invention of artificial lighting means that we can now fully control when and how much light we’re exposed to, which has altered this relationship. Today, most of us spend the majority of the day indoors, under comparatively dim artificial lights. Then, after sundown, we are exposed to more bright light, and importantly more blue light due to our digital devices. Consequently, we are getting less bright light during the day and less darkness at night.
This is important because, as noted above, light sends crucial signals to the body, and the intensity and timing of this light matters for both your health and your daily performance. But fortunately, there is a lot you can do about it. In this guide, we discuss how you can achieve a pattern of natural light and darkness in the modern world by adjusting behavior, modifying your indoor spaces, configuring your devices, and more.