The Norwegian University of Science and Technology (NTNU)
Too much or too little sleep is linked with an increased risk of certain types of cardiovascular disease. Women and the elderly are particularly at risk.
Sleeping less than
four hours or more than eight hours a night increases the risk of dying from
some types of coronary heart disease, such as heart attacks and unstable angina
pectoris, according to a study by Norwegian and Taiwanese researchers.
"This is the
single largest study that has looked at how sleep duration affects the risk of
dying from cardiovascular disease. Our results show that enough, but not too
much sleep is important for a healthy lifestyle," says first author and
postdoctoral fellow Linn Beate Strand at the Norwegian University of Science
and Technology's Department of Public Health and General Practice.
Increased risk from
too little sleep... Doctors asked 392,164 adults who came for a health check in
Taiwan between 1998 and 2011 how long they usually slept at night. Of those,
711 of the participants died of cardiovascular disease during the follow-up
period.
It turned out that the risk increased 50 per cent for participants who
slept fewer than four hours compared with participants who slept between six
and eight hours.
According to
Strand, the exact reason why too little sleep affects the risk of dying from
cardiovascular disease is still unknown, but a number of theories have been
proposed to explain the phenomenon.
Less sleep is associated with stress
responses that increase heart rate, blood pressure and secretion of adrenaline,
for example, all of which are known risk factors for cardiovascular disease.
Sleep deprivation
has also been associated with the secretion of inflammatory substances, and
previous studies have shown an association between short sleep duration,
decreased insulin sensitivity, obesity and diabetes.
... and too much
sleep isn't good either Participants who reported that they slept more than
eight hours a night also had a 53 per cent increased risk of dying from
coronary heart disease compared with participants who slept between six and
eight hours per night. After adjusting for other risk factors, the risk was
still 28 per cent higher.
"We don't know
which mechanisms can explain a direct causal link between long sleep duration
and an increased risk of dying from coronary heart disease," Strand says.
Education and
marital status were among the variables that the researchers adjusted their
analyses for, in addition to well-known risk factors such as age, sex, BMI,
systolic blood pressure, blood glucose, blood lipids, smoking, alcohol use,
physical activity level and previous cardiovascular disease or diabetes. The
increased risk could not be explained by the use of sleep medications.
Nevertheless, part
of the explanation could lie in factors that researchers lacked information on
and that could potentially be associated with both increased cardiovascular
risk and prolonged sleep, such as unemployment, low socioeconomic status,
depression and sleep apnea.
No increased risk
was associated with sleeping between four and six hours a night as compared to
sleeping between six and eight hours.
Furthermore,
researchers found that the increased risks of both short and long sleep
duration primarily affected women. The relationship between sleep duration and
the risk of dying from cardiovascular disease was also slightly stronger in the
65 years and older population than in younger individuals.
"Women are
more prone to sleep problems than men, and previous studies have also shown
that women who sleep a little or a lot may be at increased risk for
cardiovascular disease," says Strand. She emphasizes that the findings
should be interpreted with caution and do not necessarily mean that short or
long sleep duration is more dangerous for women than for men.
