Longevity Briefs: Shorter Sleep, Shorter Life?

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem: It’s no secret that lack of sleep is associated with poorer health and increased mortality. Yet while short sleep duration might shorten your lifespan, many health conditions and lifestyle practices like diet and exercise directly affect sleep. Does lack of sleep really shorten your lifespan or does poor health lead to both lack of sleep and reduced lifespan (or is it a combination of both)? To answer this question for sure, we’d need to randomly assign people to have their sleep shortened and record how long they lived, which would obviously not be ethical or practical.

Fortunately, there’s a way of getting around this problem: we can let genetics do the random assignment for us. Certain genetic variants predispose people to sleeping for longer or shorter durations, and these genetic variants can be assumed to be randomly distributed throughout the population. Why is this helpful? If people who get less sleep also die more from cardiovascular disease, this could be because poor cardiovascular health impairs sleep. However, if people with a genetic predisposition for short sleep die more from cardiovascular disease, we can reasonably assume that this was caused by short sleep, since poor cardiovascular health or the lifestyle factors that cause it cannot change people’s genes. Establishing a causal relationship in this way is called Mendelian randomisation.

The discovery: In this study , researchers analysed the relationship between genetic predictors of sleep duration and lifespan in over 1 million participants of the UK biobank. They found that genetically predicted short sleep was significantly associated with shorter lifespan. What’s more, they found that higher incidence of coronary artery disease, type 2 diabetes and depression were partially responsible for this.

Genetic predictors of sleep chronotype (being a ‘morning person’ or ‘lark’ vs an ‘evening person’ or ‘owl’) had no significant effect on lifespan, nor did genetic predictors of long sleep duration. Surprisingly, genetic predisposition for insomnia (defined as difficulty getting to sleep or frequently waking up during the night) also wasn’t significantly associated with shorter lifespan. While this may seem odd, it’s possible that people with insomnia compensated in other ways. For example, they may seek treatment or take naps during the day, while short sleepers may be less aware there is anything wrong with their sleep patterns.

The implications: This study provides support for the theory that short sleep could cause shorter lifespans, but also disagrees with previous research on some other topics. Previous studies have suggested that “evening-type” people (“owls”) have shorter lifespans on average, and a few other Mendelian randomisation studies found that genetic variants for both short and long sleep durations were linked to a significantly shorter lifespan. Meta-analyses of studies on sleep duration are also mixed on whether the relationship between sleep duration and mortality is U-shaped (meaning both too little and too much sleep are bad) or not. The present study was larger in size than the previous Mendelian randomisation studies. However, the authors of the present study note that they relied on a smaller number of genetic predictors for long sleep duration and insomnia than for short sleep duration, which could have resulted in a significant relationship going undetected. 

It’s also worth noting that while these kinds of studies may suggest that short sleep causes shorter lifespans, they can’t tell us how direct the link is. For example, people who sleep less may get more coronary artery disease in part because they do less exercise because they have less energy. Likewise, when studies find that evening people have higher mortality rates than morning people, this could be because modern society and work hours are misaligned with the circadian rhythms of evening-type people, not because there is anything inherently unhealthy about having an evening chronotype.