Sleep: The Goldilocks of Cardiology

How sleep duration raises the risk of heart attack

Sleep. That time that Shakespeare called “Nature’s soft nurse” is seen as a period of regeneration for the body. But new research lead by Iyas Daghlas from Dr. Celine Vetter’s lab at the University of Colorado suggests that another Shakespearean quote might better capture the effects of sleep: “can one desire too much of a good thing?”. According to Dr. Vetter’s group the answer is yes. Too much sleep can be bad for your heart.

Too little. Too much. Just right.

Participants in the study came from an ongoing prospective population-based cohort study in the United Kingdom called the “UK Biobank”. Over 500,000 people between 40 and 69 years of age were enrolled in the UK Biobank between 2006 and 2010. The sleep patterns of 461,347 UK Biobank volunteers were examined for this study, and a subset of 310,917 people were included for genetic analysis.

Volunteers in the study self-reported their sleep patterns. Participants were identified as ‘short’ duration sleepers if they slept an average of 4-6 hours per night, and ‘long’ duration sleepers were classified as those who got an average of 9-11 hours sleep. ‘Favorable sleep’ was between 6 and 9 hours.

Sleep Kills

Vetter’s group found that both short and long durations of sleep raised the risk for a heart attack. For short duration sleepers, the risk was 52-96% higher. On the other hand, long duration sleepers had up to a 278% greater risk of heart attack than those who slept 6-9 hours. These higher risks are not tied to sleep problems like insomnia or known cardiovascular risks like smoking, body composition or exercise habits. This means that simply sleeping less than 6 hours or more than 9 hours on a regular basis is enough to increase the risk of a heart attack.

Sleep Therapy

Heart disease is tied to a number of genetic changes that alter one’s risk independent of behavioral activities like exercise and diet. Interestingly, Vetter’s study showed that getting regular sleep of 6-9 hours reduced the risk of heart disease in people with known genetic risk factors. In other words, getting a good night’s sleep reduces the risk of a heart attack.

A Missing Link?

Approximately 35% of people worldwide sleep less than the minimum recommendation of 7 hours per night. Many studies have found that short duration or disrupted sleep increases the risk of heart disease and mortality. However, short duration sleep often occurs alongside other health conditions including obesity, diabetes and mental health issues. Do poor sleep patterns lead to cardiovascular disease through these known risk factors, or does sleep itself directly impact the risk of cardiovascular disease? Vetter’s study suggests a direct link, but they are unable to identify a mechanism.

Sleep is an important time for the body to restore itself and to repair small amounts of damage that occur with everyday life. Without sufficient sleep, these periods of restoration and repair are reduced. For example, during sleep blood pressure tends to decrease. This relieves some pressure from the cardiovascular system. If sleep duration is shortened, this period of rest is reduced and the body has less time to recover from daily activity.

Work lead by Dr. Tami Martino’s laboratory at the University of Guelph, which included our own research team, found that an acute 5-day disruption in sleep patterns had a negative effect on heart attack recovery in mice. The period of sleep disruption was too short to cause obesity, diabetes or other factors that negatively impact the heart. Instead, sleep disruption directly impaired the ability of the immune system to repair the heart after a heart attack.

The link between long duration sleep and heart attacks is not well understood. Mental health issues like depression can prolong sleep duration, and depression is a known risk factor for heart disease. Whether prolonged sleep duration from depression or depression itself is the instigating factor driving heart disease is not known. Vetter and colleagues themselves note a need to investigate this temporal relationship.

Limiting Factors

The UK Biobank study is limited to participants who live in the United Kingdom. While the United Kingdom includes people from many parts of the world, the cohort used in parts of this study lacks the diversity that is reflective of the country and the larger global population. For the investigation involving people with known a genetic risk for heart attacks, only participants with ‘White British Ancestry’ were included. The focus on a single ethnic group allows researchers to limit genetic variability that may complicate the interpretation of results. However, it also limits the ability to apply study findings to other groups. Whether the relationship between favorable sleep duration and decreased heart disease risk can be universally applied across ethnic and racial groups remains unexamined.

Goldilocks Principle

Hamlet described sleep as a way to “end the heart-ache”. But it turns out that the truth may be less like a Shakespearean tragedy and more like a fairy tale: too little sleep is bad for your health and too much can also harm. But if it is just right, everyone will live happily ever after.