On average, a New Zealander dies of heart disease every 90 minutes. Meanwhile, rates of diabetes are soaring, with the Ministry of Health stating that it is “the largest and fastest growing health issue we face in New Zealand”.
Although most efforts to combat these diseases are focused around dietary changes and exercise, chronic sleep deprivation, which is now commonplace across developed nations, is an overlooked risk factor for heart disease and diabetes.
Poor sleep places stress on the cardiovascular system in several ways, putting individuals at a greater risk of heart diseases. During my 13 years in the New Zealand education system, there were countless lessons about which food we should be eating or the importance of exercise, but sleep was seldom mentioned.
While the ways in which sleep deprivation takes its toll on the cardiovascular system are numerous (Nagai et al., 2010), these effects seem to stem from a common source; it causes the sympathetic nervous system to become overactive. The sympathetic nervous system evolved to enable humans to act quickly in response to immediate threats (this is commonly referred to as the fight or flight response.)
It does so by putting certain areas of the body such as the heart into overdrive while starving other areas of blood and oxygen. While this may be the appropriate response to a life-threatening danger such as a fire alarm during a house fire, having a chronically active sympathetic nervous system places a huge burden on the cardiovascular system.
The sympathetic nervous system activity, due to the lack of sleep causes high blood pressure, putting a strain on the walls of your arteries (Tochikubo et al., 1996). During sleep your heart rate and blood pressure drop, giving your cardiovascular system a chance to recover. When failing to sleep sufficiently, you deprive your body of this opportunity. Several large epidemiological studies and one case-controlled study have found a correlation between short sleep duration and heart attacks, even when controlling for other factors such as smoking, age and BMI. For example, a study which followed a cohort of over 70,000 women over ten years, none of which had coronary heart disease at baseline, found that sleeping five hours or less was associated with a 45% increase in the risk of suffering a heart attack (Ayas et al., 2003).
A case-controlled study examining men ages between 49-79 in Japan found that sleeping five hours or less was associated with a 200-300% increase in the risk of a heart attack (Liu et al., 2002).
You may be surprised to learn that even a modest reduction in sleep can have a huge toll on the body. Most people consider the loss of an hour of sleep to be inconsequential. However, the data suggests otherwise. Take the annual experiment involving over 1.5 billion people, across 70 countries, known as Daylight Savings. Chances are you have taken part in this experiment yourself. The careful examination of over 42,000 hospital admissions revealed that the day following the beginning of Daylight Savings when people lose an extra hour of sleep opportunity, heart attacks increased by 24%. When Daylight Savings ended, heart attacks subsequently dropped by 21% (Sandhu et al., 2014).
Insufficient sleep can also cause calcification of the arteries, increasing the risk of coronary heart disease and stroke. A study published in the European Heart Journal revealed predictions of cardiovascular outcomes via sleep durations, based on the analysis of 15 medical studies over 25 years, involving nearly half a million people (Cappuccio et al., 2011). They found that those who slept less than 6 hours per night had a 48% greater chance of developing or dying from coronary heart disease and a 15% greater chance of suffering or dying from a stroke. Their findings were consistent with numerous other studies. For example, another study conducted at the University of Chicago followed 494 mid-aged adults for 5 years. They tracked the health of the arteries of these individuals all the while examining their sleep. They concluded that if you slept 5-6 hours per night you were 200-300% more likely to suffer calcification of the coronary arteries than those sleeping 7-8 hours per night (King et al., 2008). This is particularly concerning when you consider that heart disease is the 2nd biggest killer in New Zealand according to the Ministry of Health data in 2015. Moreover, cardiovascular diseases are still the leading of death, accounting for 17.79 million deaths globally in 2017 (Ritchie & Roser, 2018).
It may surprise you that insufficient sleep also increases one's risk of diabetes both directly and indirectly. Insulin is the hormone that allows the body to regulate blood glucose level. However, individuals with Diabetes either fail to produce enough insulin, or their cells develop a resistance to insulin, causing a dangerously high level of glucose remain circulating in the blood stream. High levels of glucose in the bloodstream can damage your tissues, including heart and blood vessels, eye and kidney damage, as well as put you at a greater risk of a host of pathological conditions. Several case studies indicated the relationship between sleep deprivation and insulin resistance in healthy subjects. The lack of sleep (4 hours) increases the risk of type 2 diabetes, compared with a normal sleeping time (8 hours) (Donga et al., 2010; Rao et al., 2015).
Another study generated a U-shape relationship of sleep duration and type 2 diabetes by meta-analysis, based on a total of 18,443 type 2 diabetes patients among 482,502 participants (Shan et al., 2015). They found that those who slept less than 7 hours or greater than 9 hours per night were at an increasingly higher risk of diabetes compared with those who slept 7-8 hours per night.
Although it is important to note, analysis such as this one do not show which of the variables is causal. In fact, we have good reason to believe that those with type 2 diabetes could sleep less than healthy individuals. When the body cannot regulate the blood glucose level, the excess sugar will be excreted by urinating. Because of this, those with type 2 diabetes often urinate more frequently than healthy individuals, which could cause them to wake in the night to go to the bathroom. However, we now have evidence to suggest that this relationship is bidirectional. Short sleep is a causal factor and even a precursor to type 2 diabetes. One study found that just a single night of partial sleep deprivation (reduced from 8 hours to 4 hours) caused a 25% increase in insulin resistance (Donga et al., 2010). Another study found that just a single week of restricting sleep to 5 hours increased insulin resistance by as much as 40% in healthy men (Buxton et al., 2010). As Matthew Walker rightfully points out in his book Why We Sleep, with a change that dramatic, an otherwise healthy individual, could go to their GP following a week of sleep deprivation and be diagnosed as pre-diabetic.
Sleep has a more indirect, but equally dangerous method of increasing our risk of diabetes, as well as expanding your waistline. Short sleep impacts the hormones which regulate our appetite. Leptin is a hormone which causes us to feel satisfied following a meal. Ghrelin, on the other hand, is the hormone responsible for making us feel hungry. Short sleep increases levels of Ghrelin causing one to feel hungrier, while also suppressing Leptin, making it difficult to feel full. This is a dangerous combination.
“There is no doubt that insufficient sleep promotes hunger and appetite, which can cause excessive food intake resulting in weight gain”, according to Eve Van Cauter, the director of the Sleep, Metabolism and Health Center at the University of Chicago.
When considering either of these effects in isolation, it is clear that sleep has an impact on one's risk of developing diabetes, but when considering these effects in conjunction with one and other it begins to become clear just how damaging poor sleep is to one’s health.
With all of this in mind you may, ironically, find it difficult to sleep at night. But I am hopeful for the future. I believe sleep is currently an incredibly effective weapon in the fight against these chronic diseases, which is currently under-utilised. A recent study found that more than a third of kiwis sleep less than 7 hours of sleep per night (Lee et al., 2019). This indicates that we have some room for improvement. In the last couple of decades our understanding of sleep and its benefits has increased hugely, and as it continues to grow, I am optimistic that it will spark change both to our health care system, and our culture surrounding sleep.
Ayas, N. T., White, D. P., Al-Delaimy, W. K., Manson, J. E., Stampfer, M. J., Speizer, F. E., Patel, S., & Hu, F. B. (2003). A prospective study of self-reported sleep duration and incident diabetes in women. Diabetes Care, 26(2), 380–384. https://doi.org/10.2337/diacare.26.2.380
Buxton, O. M., Pavlova, M., Reid, E. W., Wang, W., Simonson, D. C., & Adler, G. K. (2010). Sleep restriction for 1 week reduces insulin sensitivity in healthy men. Diabetes, 59(9), 2126–2133. https://doi.org/10.2337/db09-0699
Cappuccio, F. P., Cooper, D., D’Elia, L., Strazzullo, P., & Miller, M. A. (2011). Sleep duration predicts cardiovascular outcomes: A systematic review and meta-analysis of prospective studies. European Heart Journal, 32(12), 1484–1492. https://doi.org/10.1093/eurheartj/ehr007
Donga, E., van Dijk, M., van Dijk, J. G., Biermasz, N. R., Lammers, G.-J., van Kralingen, K. W., Corssmit, E. P. M., & Romijn, J. A. (2010). A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects. The Journal of Clinical Endocrinology and Metabolism, 95(6), 2963–2968. https://doi.org/10.1210/jc.2009-2430
King, C. R., Knutson, K. L., Rathouz, P. J., Sidney, S., Liu, K., & Lauderdale, D. S. (2008). Short sleep duration and incident coronary artery calcification. JAMA : The Journal of the American Medical Association, 300(24), 2859–2866. https://doi.org/10.1001/jama.2008.867
Lee C., Sibley C.G. (2019) Sleep duration and psychological well-being among New Zealander. Sleep Health, Volume 5, Issue 6, 606-614. doi.org/10.1016/j.sleh.2019.06.008.
Liu, Y., Tanaka, H., & Fukuoka Heart Study Group. (2002). Overtime work, insufficient sleep, and risk of non-fatal acute myocardial infarction in Japanese men. Occupational and Environmental Medicine, 59(7), 447–451. https://doi.org/10.1136/oem.59.7.447
Nagai, M., Hoshide, S., & Kario, K. (2010). Sleep Duration as a Risk Factor for Cardiovascular Disease- a Review of the Recent Literature. Current Cardiology Reviews, 6(1), 54–61 https://doi.org/10.2174/157340310790231635
Rao, M. N., Neylan, T. C., Grunfeld, C., Mulligan, K., Schambelan, M., & Schwarz, J.-M. (2015). Subchronic sleep restriction causes tissue-specific insulin resistance. The Journal of Clinical Endocrinology and Metabolism, 100(4), 1664–1671. https://doi.org/10.1210/jc.2014-3911
Ritchie, H., & Roser, M. (2018). Causes of Death. Our World in Data. https://ourworldindata.org/causes-of-death
Sandhu, A., Seth, M., & Gurm, H. S. (2014). Daylight savings time and myocardial infarction. Open Heart, 1(1), e000019. https://doi.org/10.1136/openhrt-2013-000019
Shan, Z., Ma, H., Xie, M., Yan, P., Guo, Y., Bao, W., Rong, Y., Jackson, C. L., Hu, F. B., & Liu, L. (2015). Sleep duration and risk of type 2 diabetes: A meta-analysis of prospective studies. Diabetes Care, 38(3), 529–537. https://doi.org/10.2337/dc14-2073
Tochikubo, O., Ikeda, A., Miyajima, E., & Ishii, M. (1996). Effects of insufficient sleep on blood pressure monitored by a new multibiomedical recorder. Hypertension (Dallas, Tex.: 1979), 27(6), 1318–1324. https://doi.org/10.1161/01.hyp.27.6.1318