Although many studies have been performed on the link between cardiovascular disease and the duration of sleep, scientists have never really had a detailed look at what role bedtime plays.

New research published in the peer-reviewed European Heart Journal has now connected bedtime with heart health.

*Image Credit: New Africa via Shutterstock / HDR tune by Universal-Sci*

Our circadian cycle/rhythm, driven by a circadian biological clock, is a natural process that controls our sleep cycle. It repeats itself every 24 hours (approximately) and is essential for our mental and physical working.

The results of this new research show that the aforementioned circadian rhythm is likely to be disturbed by late or early bedtimes with negative effects on heart health.

The study

The research relies on objectively measured sleep onset rather than self-reports by utilizing wrist-worn accelerometers.

Almost 90.000 participants (aged 43 to 79 years old) took part in the study. Information regarding at what time participants fell asleep and at what time they woke up was gathered over a period of 7 days.

After the measurements took place, participants received follow-up assessments. Through an average follow-up period of 5.7 years, 3,172 participants (3.6%) developed cardiovascular disease.

The results

After adjusting for other relevant factors such as sleep duration, sleep irregularity, sex, age, smoking status, BMI, blood pressure, etc, cardiovascular disease occurred most often with those who fell asleep at or after midnight and the least often with those who fell asleep between 10 pm and 11 pm.

Overall, the study shows that people who fall asleep after midnight have a 25% increased risk of developing cardiovascular diseases compared to those who fall asleep between 10 and 11 pm. Interestingly, falling asleep too earlier (before 10 pm) is nearly as bad, with a heightened risk of 24% versus a sleep onset between 10 and 11 pm. Finally, those who fall asleep between 11 pm and midnight have a 12% greater risk versus 10 and 11 pm. So, all in all, the riskiest time to go to sleep is after midnight.

Falling asleep too early may also be bad - *Image Credit: fizkes via Shutterstock / HDR tune by Universal-Sci*

Above mentioned numbers mainly apply to women where only sleep onset at an earlier time (before 10 pm) indicated significant adverse effects on heart health for men.

Dr. David Plans, one of the study's authors, postulates that the adverse health effects of going to bed after midnight may be due to the fact that it reduces the chance of seeing morning light, which has an important factor in resetting your biological clock.

With regard to the difference in effect between men and women, Dr. Plans noted that there might be a difference between men and women in how the endocrine system reacts to a disturbance in circadian rhythm.

He concluded by stating that although the research results do not attest causality, the timing of sleep has emerged as a possible cardiac risk factor – unattached from other risk factors and sleep traits.

Therefore, if the results get confirmed by additional research, basic sleep hygiene and sleep timing may prove to be low-cost public health targets for reducing the risk of heart disease.

Sources and further reading on the subject of sleep: