When it comes to sleep, Goldilocks may have argued the trick is to find a bed that is “just right” but research suggests there may also be an optimal time to nod off – at least when it comes to heart health. Not too early and not too late, with the sweet spot being between 10pm and 11pm.
The study, based on data from more than 88,000 participants of the UK Biobank, suggests going to sleep at 10pm or shortly after is associated with a lower risk of developing cardiovascular disease compared with falling asleep earlier or later at night.
The study cannot prove that early or late bedtimes contribute to the development of cardiovascular disease – not least since when people fall asleep may be linked to other health conditions or behaviours, such as staying out late and drinking, that may be linked to cardiovascular disease risk. Moreover, the study did not look closely at the quality of the participants’ sleep, only length and timing.
However, Dr David Plans, the head of research at Huma Therapeutics and senior lecturer at the University of Exeter, who co-authored the study, said one possibility was that early or late bedtimes might result in individuals missing important cues such as morning daylight, which help to reset the body’s internal clock each day.
If the body clock is not reset properly over a long period of time, “that misalignment of behaviours and the circadian clock increases inflammation and can impair glucose regulation, both of which can increase risk of cardiovascular disease”, Plans said.
Writing in the journal European Heart Journal – Digital Health, Plans and colleagues report that 3,172 of the 88,026 participants, recruited between 2006 and 2010, developed cardiovascular disease over an average follow-up time of 5.7 years, none of whom had the condition, or a sleep disorder, at the start of the study.
The team then drew on data from wrist-based devices worn by participants for seven days to explore whether there was an association with the time that participants fell asleep at night.
The team found that of the 3,172 participants, 1,371 fell asleep after midnight on average over the seven days of device wearing, 1,196 nodded off in the hour from 11pm, and 473 fell asleep in the hour from 10pm. Only 132 dozed off before 10pm.
After taking into account various information, such as participants’ age, sex, smoking status, duration of sleep, sleep irregularity, whether they had diabetes, their blood pressure and socioeconomic status, the researchers found participants who fell asleep between 10pm and 10:59pm had a lower risk of cardiovascular disease than those who dozed off earlier or later.
More specifically, those who fell asleep at midnight or later had a 25% higher risk of going on to develop cardiovascular disease, while those who fell asleep before 10pm had a 24% increased risk. Even nodding off an hour later was linked to a difference – those falling asleep between 11pm and 11:59pm had a 12% greater risk of cardiovascular disease than those who fell asleep in the hour before.
“Because we also adjusted for all of the other more common cardiovascular risk factors, it’s clear that this association is significant in some way,” said Plans.
The team say the findings appear to be stronger in women than men, although the reasons for this remain unclear.
The study has limitations, including that it is based only on data from adults aged 43 to 79, and participants of the UK Biobank – a database of genetic and lifestyle information that researchers are using to investigate myriad health issues – are predominantly white. Plans said further research, with larger numbers of participants, is needed to examine the findings, adding there was not enough evidence at present to prescribe a particular bedtime to the public.
However, he said the study added support to the importance of sleep hygiene – habits that help with a good night’s rest.
“People often assume that cardiovascular disease is a consequence of physiological influences,” Plans said. “Whereas actually, the behavioural influence on the cardiovascular system as a result of circadian disruption is enormous.”