Irregular sleep-wake cycle linked to heightened risk of major cardiovascular events

An irregular sleep-wake cycle is associated with a heightened risk of major cardiovascular events, such as heart attack and stroke, even for those who clock up the recommended nightly hours of shut-eye, finds research published online in the Journal of Epidemiology & Community Health.

Most studies looking at the impact of sleep on health have focused on sleep length and less is known about the impact of sleep patterns, in particular the impact of irregular sleep—defined as variations in the time a person goes to sleep and wakes up. 

To explore this further, the researchers drew on 72,269 people aged 40 to 79, taking part in the UK Biobank study, none of whom had any history of major cardiovascular events.

They wore an activity tracker for 7 days to record their sleep, the data from which were used to calculate each person's Sleep Regularity Index (SRI) score. 

People with an SRI score of more than 87 were considered to have a regular sleep pattern, while those with an SRI score of less than 72 were categorised as irregular sleepers. And those whose scores fell between this range were regarded as moderately irregular sleepers.

Incidents of cardiovascular death, heart attack, heart failure, and stroke over the next 8 years were collected from death registries and hospital records and used to calculate the risk of these events for each sleep pattern group.

After accounting for a range of potentially influential factors, such as age, physical activity levels, discretionary screen time, fruit, vegetable, and coffee intake, alcohol consumption, smoking, mental health issues, medication use, and shift work, irregular sleepers were 26% more likely to have a major cardiovascular event than those with a regular sleep wake-cycle, while moderately irregular sleepers were 8% more likely to do so. 

More detailed analysis of the data showed that the relationship between decreasing SRI score and higher risk of events was almost linear, with a steeper reduction in risk at higher (better) SRI scores.

The recommended amount of nightly sleep is 7 to 9 hours for 18 to 64 year olds and 7 to 8 hours for those aged 65 and above. A greater proportion of regular sleepers met the recommended sleep quota than irregular sleepers: 61% vs 48%.

But when the impact of clocking up the recommended sleep quota was taken into account, the risk of a major cardiovascular event wasn’t offset in irregular sleepers who achieved this, although it was in moderately irregular sleepers. 

This is an observational study, and as such, can’t establish cause and effect, and the researchers acknowledge various imitations to their findings. 

While the study’s sample size was large, the UK Biobank may not accurately reflect the UK population. Sleep patterns were also assessed during a single week only. The activity tracker didn’t distinguish between quiet wakefulness and sleep, and the sleep algorithm used to calculate SRI scores didn’t take napping into account, only the longest period of sleep. 

But the researchers nevertheless conclude that the findings suggest that irregular sleep is strongly associated with a risk of major adverse cardiovascular events in adults, irrespective of whether or not recommended sleep quotas are met.

“More importantly, our results suggest that sleep regularity may be more relevant than sufficient sleep duration in modulating MACE [major adverse cardiovascular event] risk,” they say.

They add: “Findings from this study suggest that more attention needs to be paid to sleep regularity in public health guidelines and clinical practice due to its potential role in cardiovascular health.”