April 2020, ScientificAmerican.com 25THE SCIENCE
OF HEALTHClaudia Wallis is an award-winning science journalist whose
work has appeared in the New York Times, Time, Fortune and the
New Republic. She was science editor at Time and managing editor
of Scientific American Mind.Picture, if you will, your typical sleep apnea sufferer. Chances
are he is middle-aged and overweight and snores like a freight
train. Note the male pronoun. Twenty-five years ago experts be -
lieved that the condition, in which breathing is disrupted dur-
ing sleep, was about 10 times as common in men as in women.
Better-quality studies have since reduced that ratio to roughly
three to one, but as more data come to light, it is becoming
clearer that sleep apnea—and the broader category known as
sleep disordered breathing—simply looks a little different in
women. And that suggests it is often overlooked.
Sleep apnea is a concern because it raises the risk of heart
attacks, hypertension, arrhythmias, insulin resistance, strokes and
accidents that result from daytime sleepiness. Put simply, gasping
for breath at night and not giving your body a thorough rest puts
a lot of pressure on the cardiovascular system, raises adrenaline
levels and ignites inflammation. Doctors diagnose apnea with a
sleep test, often done at home, that measures your apnea-hypop-
nea index. This index reflects the average number of times an
hour that you have an episode lasting at least 10 seconds during
which breathing stops (apnea) or becomes so deficient that blood
oxygen levels fall by 3 or 4 percent or more (hypopnea). Fewer
than five such episodes an hour is considered normal. Five to 15 is
mild sleep apnea, 15 to 30 is moderate and more than 30 is severe.
Most home tests do not, however, examine the stage of sleep in
which these episodes occur, and that may be a problem. A grow-
ing body of evidence shows that for many women disrupted breath-
ing is concentrated in the rapid eye movement (REM) phase, which
is also when dreams are most vivid and when heart and respirato-
ry rates become less regular. A study of 2,057 men and women aged
45 to 84, published last November in the journal Sleep, found that
women have just as many events as men do during the REM phase.
“Whatever protection women have in non-REM sleep is not there
during REM sleep,” says Christine Won, medical director of the Yale
Centers for Sleep Medicine and lead study author. Disrupting REM
may be especially bad for health. “Studies suggest that how many
events you have during REM sleep is what really puts you at risk
for cardiovascular health effects,” Won says. But because REM ac-
counts for only about 20 percent of a person’s nightly slumber, a test
that averages events across the entire night can be misleading.
Several other sex differences emerged from the new study. “One
of our findings is that women have a lower arousal threshold—they
are more likely to wake up at night in response to a given apnea,”
says Susan Redline, senior author of the study and a senior physi-
cian at the Division of Sleep and Circadian Disorders at Brigham
and Women’s Hospital in Boston. This may also impact diagno-
sis. Women are more likely than men to briefly wake up before
their oxygen level falls by more than 4 percent—the threshold used
to identify and treat sleep apnea in patients covered by Medicare.
When Won and Redline used a cutoff of 3 percent oxygen desat-
uration, many more women met the criteria for sleep apnea.
These differing patterns may help explain why women with
sleep apnea are more likely to complain about morning headaches,
fatigue, depressed mood and insomnia. In men, a big complaint
(usually from a bed partner) is loud snoring, along with daytime
sleepiness. In both sexes, apnea rates rise with obesity and age.
Hormones most likely play a role in these sex differences. Sleep
apnea increases in women after menopause, and it is common in
women with polycystic ovary syndrome, a condition character-
ized by high levels of testosterone. Redline notes that anatomy
may also be a factor: “Men have a longer and more collapsible air-
way.” As for the female tendency to wake up more easily, evolu-
tionary pressures and experience might be at work: “It may be that
women evolved to wake up to tend to their children,” Redline says.
The leading therapy for apnea is the use of a CPAP (continu-
ous positive airway pressure) machine to force air into the throat,
keeping the airway open. Research suggests that it works equal-
ly well for REM and non-REM apnea. But as scientists shed more
light on the varying patterns of the condition, treatment may be-
come more tailored to the individual. Diagnostic criteria might
also need to change to capture more cases in women. Medicare’s
4 percent desaturation threshold is one example. And the increas-
ing use of at-home testing rather than costlier testing in a sleep
lab may be another. “If our findings are true,” Won says, “then
home sleep apnea testing biases against diagnosing women.”Of Sex and
Sleep Apnea
The risky disorder often looks different
in women and may get ignored
By Claudia Wallis
Illustration by Fatinha Ramos
© 2020 Scientific American