National Heart Month: How does sleep affect heart health?

Our hearts do a pretty remarkable job. Your heart will probably beat over 100,000 times today, pumping blood and essential supplies to every organ in the body. It will beat over 35 million times this year, and at least a few billion times in your lifetime.

I’ve previously written about some of the ways that sleep can protect against heart disease.

In this article I’ll explain in more detail why it’s not just the amount of sleep which is important, it’s also when we get it.

What happens to the heart during sleep?

• During deep, slow wave sleep, your heart rate slows by as much as 20-30%. This is a very physically restorative stage of sleep. Growth hormone is produced, which helps damaged muscle cells in your body to repair. Heart or cardiac muscles are much slower to repair but we know that younger adults can renew about 1 percent of their heart cells per year. Older adults can only replace an estimated 0.45% of their cardiac muscle per year.

• Blood pressure falls by 10-15% overnight as the ‘fight or flight’ stress hormones drop to their lowest levels, and the ‘rest and digest’ nervous system takes over. If this overnight nocturnal dipping in blood pressure doesn’t happen, for example if you’re working at night, there is an increased risk of heart disease.

• Heart disease is accelerated by chronic inflammation, when the body’s immune causes damage to its own tissues and cells, including the blood vessels supplying the heart, leading to atherosclerosis. Sleep has anti-inflammatory effects, and sleep deprivation has been linked to increased levels of inflammatory markers.

Why is the timing of sleep so important for heart health?

The timing of sleep is controlled by our circadian rhythms, 24 hour patterns of activity which are written into the DNA of every cell in the body. Circadian rhythms don’t only influence when we feel sleepy, they also govern appetite, mood, blood pressure, stress hormones and immune function – in fact, every physiological system runs on a 24 hour clock.

Heart attacks and strokes are most common in the early morning when we naturally see a daily peak in levels of the stress hormone, cortisol, and blood clotting factors. If stress levels are particularly high, or you’re very short of sleep (such as after daylight savings time), vulnerability to blood clotting events increases.

When we wake up and fall asleep at the same time each day, according to a regular 24 hour rhythm, all of our internal processes operate in sync, working together. We wake up easily and feel sleepy at a similar time each night.

Irregular sleep-wake patterns can upset our circadian rhythms and increase the risk of disease (Chellappa 2019). Recent research has shown that even small fluctuation in sleep-wake patterns is linked to early signs of atherosclerosis – the fatty deposits which can interrupt the blood supply to the heart. One study in healthy people who tracked their sleep over 7 days found that the less variability in their sleep timing, the lower the risk of heart disease over 5 years (Huang 2020).

What’s the best time to go to bed for a healthy heart?

If sleeping to match your internal circadian rhythms is important, could it be that falling asleep at a particular time can help to protect the heart?

A recent study by Nikbakhtian and colleagues analysed the bedtimes of 88,000 people in the UK who wore a tracker for 7 days to record the timing and duration of their sleep. The average age of people in the study was 61, ranging from 43 to 79 years.

Over 6 years, 3,000 of the group being tracked had a new diagnosis of heart disease.

The risk of heart disease was lowest for those people with a bedtime between 10 and 11pm.

In fact, even after accounting for differences in age, sleep duration and lifestyle, there was a 25% higher risk of cardiovascular disease for those who fell asleep at midnight or later, a 12% greater risk for falling asleep between 11pm and midnight, and a 24% raised risk for falling asleep before 10pm. The association with increased heart disease risk was stronger in women, with only sleep onset before 10pm remaining a significant risk for men.

The authors of the study were not able to explain why 10 to 11pm was an optimal bedtime. We have more deep sleep in the first part of the night, and it may be that delaying bedtime interferes with getting enough of this restorative sleep stage.

Other studies have also found that late bedtimes during the week are associated with increased risks of heart disease. However, more research is needed to explain why falling asleep before 10pm might have a negative impact on heart health.

Could light at night influence sleep and heart health?

Another way to support healthy circadian rhythms is to keep your bedroom dark throughout the night using blackout blinds, and/or to wear an eye mask.

A recent American study recruited over 500 older adults between the ages of 63 and 84 (Kim et al 2022). They wore a wearable monitor to track their sleep patterns, and also to measure the amount of light they were exposed to during the night. The study also measured their risk of traditional cardiac risk factors such as obesity and diabetes.

Keeping a low light on during the night was associated with increased risks of obesity, high blood pressure and double the risk of diabetes, compared to older people sleeping in the dark. Adults sleeping in dark rooms had better quality sleep and were more active during the day.

A number of studies have found that keeping a light on in the bedroom keeps the body in a higher state of alertness than sleeping in a completely dark room. If you have to keep a light on, aim to get plenty of exposure to bright light during the day, which is thought to reduce your sensitivity to light at night.

What are the key takeaways for heart health?

• Aim to get ready for bed at a similar time each night, ideally switching out the light between 10 and 11pm.
• Aim to wake up at a similar time each morning, as often as possible. 
• If you can’t stick to a regular routine, for example, because of shift work, then other ways of protecting the heart become even more important, such as regular physical activity, tackling sources of stress and meditation.
• To reduce the amount of light in the bedroom, switch off lamps, computers, tablets, cell phones, and other light-emitting electronics before you go to bed. Keep a torch or a dim light near the bed in case you need to get up at night.

Read More from Dr Sophie Bostock