A new study has shown a J-shaped relationship between sleep duration and future stroke risk, with a particularly strong signal associated with long sleep duration.
Investigators at the University of Cambridge in the United Kingdom found that short sleep was associated with an 18% increased stroke risk (not statistically significant), and long sleep was associated with a 46% increase in stroke risk after adjustment for conventional cardiovascular disease risk factors and comorbidities.
In addition, those who reported persistently long sleep had double the stroke risk compared with those with persistently average sleep duration, and people whose sleep duration increased markedly over the course of a 4-year period had almost a fourfold increased risk.
The researchers also performed an updated meta-analysis of studies looking at sleep duration and stroke that showed very similar results.
"We believe long sleep duration is an indicator of increased stroke risk. We are not suggesting that sleep causes the increased risk, but it could be a useful marker of other underlying conditions that do increase stroke risk," lead author Yue Leng, MPhil, told Medscape Medical News.
"By asking one question about how many hours a patient sleeps and whether this has changed in recent years, doctors could ascertain information about that patient's risk of stroke," he added.
The study was published online February 25 in Neurology.
Sleep Pattern Change: Cause for Concern
The investigators analyzed data from the European Prospective Investigation of Cancer–Norfolk (EPIC Norfolk) cohort study. The current analysis involved 9692 stroke-free participants aged 42 to 81 years who answered a questionnaire on sleep duration in 1998 to 2000, and again in 2002 to 2004. All stroke cases were recorded until 2009.
After 9.5 years of follow-up, 346 cases of stroke had occurred. Long sleep (more than 8 hours) was significantly associated with an increased risk for stroke, with a hazard ratio of 1.46 (95% confidence interval [CI], 1.08 - 1.98) after adjustment for all covariates. The association remained robust among those without preexisting diseases and those who reported sleeping well.
The association for short sleep (less than 6 hours) was smaller (and not statistically significant), with a hazard ratio of 1.18 (95% CI, 0.91 - 1.53).
Subgroup analyses suggest that the association for short sleep was stronger among younger people, but the association for long sleep was stronger among older individuals.
For the updated meta-analysis, the authors searched Ovid Medline, EMBASE, and the Cochrane Library for prospective studies published until May 2014 and pooled effect estimates using a weighted random-effect model. Results suggested pooled hazard ratios for stroke of 1.15 (95% CI, 1.07 - 1.24) for short sleep and 1.45 (95% CI, 1.30 - 1.62) for long sleep.
Sleep/Stroke Association Probably Not Causal
Coauthor of an accompanying editorial, Alberto R. Ramos, MD, codirector, UHealth Sleep Medicine Program, University of Miami Miller School of Medicine, Florida, also believes the association between long sleep duration and increased stroke risk is probably not causal.
"There have been several studies now showing such a link. The big question is why: Is too much sleep a mechanism that causes stroke? Or is it just a marker, [and] there could be other underlying issues that causes people to sleep for longer and increase their risk of stroke? I would think it is more likely to be a marker of a confounding underlying issue than the sleep itself," he told Medscape Medical News.
For example, said Dr Ramos, sleep apnea is a condition that could cause patents to sleep longer to compensate for their disrupted sleep and is also a risk factor for stroke.
He pointed out that several studies have shown a U-shaped curve with short and long sleep durations both associated with cardiovascular risk factors such as blood pressure, diabetes, and obesity.
"While it seems to make sense [that] too little sleep could increase risks of various conditions," he said, "the longer sleep link is more puzzling, but continues to be shown in new studies."
He continued, "It might be as simple as people who spend a lot of time in bed are more likely to be unhealthy. They have less time moving around being physically active, or it is a sign that they have underlying illness."
No Measure of Sleep Quality
Dr Ramos noted that in this study, there was no measure of sleep quality, so it is very difficult to draw any conclusions.
"They just asked the participants themselves how long they slept. So that could mean they were lying in bed dozing, or they were completely asleep," he said.
More information on this observation will come from future studies including objective sleep measures.
Dr Ramos noted that several such studies are now underway using actigraphy, where subjects wear a watch-like device that records sleep patterns.
"This allows more information on sleep quality to be recorded, like polysomnography, but is more practical because patients are living normally at home rather than attending a sleep lab, which enables larger studies to be done," he said.
Dr Ramos said the large increase in stroke risks in people who had a marked change in sleep patterns from shorter to longer sleep durations was particularly interesting.
"The important thing seems to be change in sleep pattern. If someone has a big change in sleep pattern, say from 5 to 6 hours a night to 9 to 10 hours a night, then they should be evaluated for cardiovascular risk."
The EPIC-Norfolk study was supported by grants from the Medical Research Council of the United Kingdom and Cancer Research UK. The authors and editorialists have disclosed no relevant financial relationships.
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Cite this: Long Sleep Duration May Signal Increased Stroke Risk - Medscape - Feb 27, 2015.