Even if you get enough sleep… What if your sleep cycle is irregular?

Research has shown that even if you get enough sleep, if your sleep cycle (sleep-wake cycle) is irregular due to shift work, the risk of cardiovascular diseases such as heart attack and stroke can increase by up to 26%. A research team at the Children’s Hospital of Eastern Ontario (CHEO), School of Medicine, University of Ottawa, Canada, said this was the result of a survey and analysis of 72,269 people aged 40 to 79 who participated in the UK Biobank study.

Professor Jean-Philippe Chaput (pediatrics), lead author of the study, said, “Sleep irregularity refers to frequent changes in sleep schedules, such as going to bed and waking up at different times every day due to shift work. “This causes confusion in the body and has a negative impact on health,” he said. He is focusing on research on obesity prevention and lifestyle behavior improvement (improvement of sleep, physical activity, eating habits, etc.).

According to the research team, most of the previous studies examining the impact of sleep on health focused on sleep duration. There is not much research on the effects of sleep patterns, especially the effects of regular and irregular sleep due to the ‘sleep cycle’, which is the change in time between falling asleep and waking up.

The research team had participants who had never suffered from cardiovascular disease wear an activity tracker for 7 days and record their sleep data. This data was used to calculate each individual’s ‘Sleep Regularity Index (SRI)’ score. People with an SRI score of 87 or higher were classified as having a regular sleep pattern, and people with an SRI score of less than 72 were classified as having an irregular sleep pattern. People who fell between these ranges were considered to have moderately irregular sleep. The research team collected data on deaths due to cardiovascular disease and the occurrence of heart attacks, heart failure, and stroke over the next eight years from death registries and hospital records and used them for analysis.

The researchers took into account a variety of potentially influential factors, including age, physical activity level, discretionary screen time, fruit, vegetable and coffee consumption, alcohol consumption, smoking, mental health problems, drug use and shift work. Studies have shown that people with irregular sleep cycles have a 26% higher risk of major cardiovascular disease compared to people with regular sleep cycles. People with moderately irregular sleep cycles had an 8% higher risk of cardiovascular disease than those with regular sleep cycles. In particular, it was analyzed that the higher the SRI score, the more steeply the risk of cardiovascular disease decreases.

According to the research team, the recommended sleep time is 7 to 9 hours per day for people aged 18 to 64 and 7 to 8 hours per day for people aged 65 and older. This study is an observational study, so it cannot determine a causal relationship, and it has limitations such as evaluating sleep patterns for only one week. However, the research team emphasized, “This study suggests that ‘sleep regularity’ may be much more important than ‘sufficient sleep time’ in terms of controlling the risk of major cardiovascular diseases such as heart attack and stroke.”

The results of this study (Sleep regularity and major adverse cardiovascular events: a device-based prospective study in 72,269 UK adults) were published online in the Journal of Epidemiology & Community Health.