Could sleep patterns be a predictor of ill health, and if so, what does this mean for health insurers? Nicola Oliver looks at the evidence
It is widely suspected that sleep quality and duration may be associated with increased risk of disease, and could even play a role in mortality. Long sleep duration (more than nine hours) is potentially associated with increased risk of stroke, all-cause mortality, diabetes, coronary heart disease, cardiovascular disease, cognitive decline and obesity. Short sleep duration (fewer than six hours), meanwhile, could be linked to increased risk of mortality, diabetes, cardiovascular disease, stroke, cognitive decline and obesity.
It has been suggested that a lack of gold-standard, randomised controlled trials investigating the association between sleep quality and health outcomes means we should not interpret the findings of observational studies as representing any kind of causal evidence. However, more recent studies have identified that there may be a direct association between health and sleep quality through the multiple biochemical pathways, central to healthy functioning, that impact numerous body systems. So, does sleep quality require further consideration in terms of its potential as a health risk?
What is sleep?
The Oxford English Dictionary describes sleep as "a condition of body and mind which typically recurs for several hours every night, in which the nervous system is inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended."
It is governed by a complex balance of neurotransmitters that work in tandem with the circadian rhythm. This balance is subject to disruption from, among many things, light exposure - both natural and artificial - and stimulants such as caffeine.
The normal physiological demands of waking hours are reduced during sleep, but the brain is not completely inactive. As we cycle through the stages of sleep - up to four times each night - our brains are busy processing memories, managing our biochemical balance and clearing away unwanted proteins. Certain physiological activities associated with digestion, cell repair and growth are often greatest during sleep, suggesting that sleep may be important for cell repair and growth.
Stage one sleep is a very light stage of sleep, from which we can be easily roused. Stage two is more of an intermediate stage on the journey to stage three sleep, which is a deeper, restorative stage.
There are two main types of sleep:
- Non-rapid eye movement (NREM) - also known as 'quiet sleep'
- Rapid eye movement (REM) - also known as 'active sleep' or 'paradoxical sleep', during which we dream. The brainwave activity during this period resembles that of brainwave activity observed during wakefulness.
Just before we fall asleep, our bodies begin to lose heat to the environment; some researchers believe this actually helps to induce sleep. In addition, our breathing rate slows and becomes more regular, there is a reduction in heart rate and blood pressure, and renal function decreases. However, breathing and cardiovascular activity may become variable during REM sleep.
Figure 1 displays these elements across the course of a typical eight-hour duration of sleep.
It is thought that, during our waking hours, the body produces sleep-regulating substances which increase the pressure to sleep the more they accumulate. The best-known of these substances is adenosine, levels of which rise during the day as sleep debt builds, and then fall rapidly during the sleep period. Adenosine is created as a natural by-product of using up our internal energy stores.
What does research tell us?
Multiple studies have sought to examine the association between sleep quality and health outcomes, including any association with mortality. Figure 2 displays the key areas of research interest and the findings from research studies.
On the whole, these studies have been observational, relying on self-reported sleep duration and quality, and do not provide any causal evidence to link sleep quality and health. However, if we dig a little deeper, we find that there are some underlying mechanisms at play.
The key studies examining the association between night shift workers (who potentially have poorer sleep quality) and health outcomes find that those working night shifts have higher odds of obesity, higher total calorie intake, more chance of being a smoker, lower socioeconomic status, and less sleep overall. The risk profile of night shift workers may already be adverse, but even after controlling for these risk factors, a small yet significant risk for coronary heart disease persisted.
Insufficient sleep and circadian misalignment have a negative impact on glucose regulation, which can cause insulin resistance and diabetes. This could help explain some of the observed health outcomes in those with short sleep duration. Furthermore, it is understood that sleep apnoea, an obstructive respiratory condition that could also cause short sleep duration, is possibly linked to increased risk of stroke and mortality.
It is purported that, while long sleep duration in itself is a marker of poor health, it also potentially reduces an individual's levels of physical activity and exposure to natural light - both of which have their own negative effects on health and wellbeing.
The lack of gold-standard-type, randomised controlled trials (RCTs) in sleep research to help confirm any causal association between sleep and health does present a problem for some researchers. However, it should be pointed out that there are no RCTs for accepted lifestyle risk factors such as smoking - yet we fully accept that smoking is a key cause of disease and premature mortality.
It is suggested that high-density sleep EEG recordings represent the definitive method for recording sleep quality, allowing us to truly understand what is happening during sleep. Used with biomarker monitoring, brain imaging and cognitive testing, this approach may unlock the pathways that link sleep, wellness and disease.
The Center for Human Sleep Science in California, headed and founded by professor of neuroscience and psychology Matthew Walker, takes just this approach. Dr Walker endorses an eight-hour sleep requirement for everyone, based upon his findings from testing in the sleep lab at the Center. However, other specialists, such as Dr Gregg Jacobs - a behavioural sleep medicine specialist at Harvard Medical School - argue that required sleep duration varies from person to person, and that those who suffer with insomnia have different responses to sleep disruption through adaptation. Dr Jacobs also argues that worrying about not achieving a set duration of sleep can induce insomnia, thus creating a cycle of sleeplessness.
One thing that Jacobs and Walker agree on is that cognitive behavioural therapy (CBT) is the only way to treat sleep disorders such as insomnia; pharmaceutical approaches such as benzodiazepines simply induce sedation, and not natural sleep.
Is sleep a risk factor for ill health?
Observational studies consistently suggest a link between poor quality sleep and ill health. Additionally, studies conducted on healthy volunteers in sleep labs find that our biochemical profile is disrupted by disturbed and reduced sleep patterns, and it is suggested that this balance is essential for cardiovascular and neurological health.
A lack of gold-standard evidence in a large general population does not negate these lab findings. Individual requirements for sleep are likely to be more personal than the rigid eight-hour suggestion, but there is probably a minimum threshold. Similarly, people with a diagnosed sleep disorder such as insomnia may well have a protective adapted response to sleep deprivation.
Unlike cigarette smoking or other unhealthy habits, sleep quality is often outside of the control of the individual - so poor sleep as a risk factor is unlikely to become a consideration in the insurance application process. However, with the emergence of wellness insurance, understanding sleep could be beneficial - but from the perspective of providing CBT for sleep disorders as part of a wellness package, rather than penalising poor sleep.
Nicola Oliver is the founder of Medical Intelligence (UK) Ltd. She is an IFoA affiliate member, chair of the Diabetes working party, and deputy chair of the Antibiotic Resistance working party
