(Zach Meyer for The Washington Post)

Tracey Thomsen Anderson, 57, a retired ad agency copywriter from Colorado Springs, sleeps nine or 10 hours every night, and has done so her entire life.

“My ability to sleep through ridiculous circumstances was legendary as a kid — parties, fireworks, I slept through a car wreck once,” she says. “I can get by on eight for a day or two, but I feel like a zombie all day with anything less than nine.”

This may sound like heaven to the consistently sleep-deprived, but it doesn’t always seem that way to her. “I sometimes feel like I am wasting time sleeping,” she says. “I did the math once. If I live to 85, and could have slept an average of one hour less per day, that adds up to something like 1,300 extra days of living over a lifetime. That’s 3½ years — what do you think you could do with an extra 3½ years?”

Similarly, Kate (who asked that her last name not be used), a 52-year-old special-education teacher who lives in Upstate New York, would sleep 10 hours a night — if she could. But she rarely gets the chance. She wakes up every day at 5 a.m. so she can get to her job on time. “I try to be consistent about my bedtime, which is 9 p.m. most nights,” she says. “I know I should be in bed by 8 p.m., but I just have too much to do in my day.”

They are among the estimated 2 percent of the population known as “long sleepers,” people who regularly sleep more in a 24 hour period than what is usual for others in their age group. Long sleepers often sleep as much as 10 to 12 hours a night, a consistent lifelong pattern which is normal for them, and unrelated to any medical conditions, according to the American Academy of Sleep Medicine.

“It’s not considered a disorder, but a normal variant,” says Rafael Pelayo, a sleep specialist at the Stanford Center for Sleep Sciences and Medicine.

Nobody knows why some people sleep long, although researchers suspect genetics may be involved. Kate’s father was a long sleeper, as was Anderson’s father and one of her children. “My father was a long sleeper, but my mother was not. I’m pretty sure it drove her nuts to have my dad and me snoozing away half the morning,” Anderson says.

Most long sleepers initially are healthy, although they often feel short of time and out of sync with family and friends.

“It is an inconvenience at times, especially since my husband is a really early riser,” Anderson says. “I would have to go to bed right after dinner in order to wake up at his 4 a.m. time and feel rested. This puts us on totally different schedules for when each of us is feeling the most lively or productive.”

The harmful effects of sleep deprivation — less than six or seven hours a night — are well documented. They include such chronic conditions as type 2 diabetes, heart disease, obesity and depression, as well as impaired driving and work performance. And the onset of excessive sleep after a regular nightly routine of seven hours can be a sign of disease. But the health impact of a lifetime of longer-than-usual sleeping is still unclear.

“I cringe whenever I hear somebody say longer sleep is bad for you,” says Nathaniel Watson, a sleep medicine specialist in Seattle and a past president of the academy. “There is no evidence that sleep in and of itself is bad for you. There is a ton of evidence that — when you sleep-deprive people — everything — performance, mood, immunity — suffers. But there is no clear scientific evidence that extended sleep has any negative physiological consequences for healthy individuals.”

A consensus statement issued in 2015 by experts convened by the academy and the Sleep Research Society recommended at least seven hours of sleep as optimal and described sleeping more than nine hours a night as appropriate for young adults, those recovering from sleep “debt” and those who are ill. “For others, it is uncertain whether sleeping more than 9 hours per night is associated with health risk,” the statement said.

Nevertheless, not all sleep experts are convinced that long sleep is benign. They point to a body of research that links excessive sleep with a number of potentially serious health issues, although the studies don’t pinpoint lengthy sleep as the direct cause. Such studies have suggested that long sleepers (as well as short sleepers) do, in fact, face health dangers over the long term, including an increase in their death rate. One recent study also found a link between stillbirth and pregnant women who sleep an undisturbed nine or more hours a night.

(For research purposes, many studies use eight hours as the lower limit for long sleep, rather than nine or 10 hours, which greatly expands the population of long sleepers.)

“Long sleep duration has been linked with numerous health outcomes, including all-cause mortality, diabetes, cardiovascular disease, stroke, coronary heart disease and obesity,” says Neomi Shah, associate professor of medicine, pulmonary, critical care and sleep medicine at Mount Sinai’s Icahn School of Medicine. “There is also emerging evidence that suggests that long sleep duration may also be associated with Alzheimer’s disease and vascular dementia.”

Many of the studies are epidemiological in nature, that is, they track trends and distribution, and also rely on participants’ “self-reporting,” a methodology that may encourage subjects to include their time in bed, rather than in actual sleep. “The problem with epidemiological studies is that they provide associations, not causations,” says Timothy I. Morgenthaler, co-director of the Mayo Clinic’s Center for Sleep Medicine. “So, many [experts] question if perhaps there are other things associated with long sleep that are the reasons for the increased sleep, and the adverse health outcomes.”

While studies have established a link between long sleep and increased mortality, they haven’t figured out whether it’s the sleep itself — or something else — that is responsible. Shawn Youngstedt, a sleep researcher at Arizona State University and the Phoenix VA Health Care System, agrees that identifying the cause remains a challenge. Nevertheless, he is convinced the connection is real.

“There are dozens of epidemiological studies that show eight or more hours is hazardous, and that’s a big chunk of the population,” he said, citing severa l studies where the subjects were healthy at entry but still experienced an increase in their death rate. One such study, for example, followed 3,820 people in Spain between 2001 and 2008. It found more deaths among those who slept between eight and 11 or more hours — including those whose health status was good — than among those who slept seven hours.

“This belies the argument that associations of long sleep with mortality can be explained simply by poor health or moribund conditions leading to long sleep,” Youngstedt says.

With sleep deprivation, “it’s easy to show experimentally it has negative effects right away, whereas it’s probably harder to demonstrate that experimentally with longer sleep,” he said. “But there’s enough circumstantial evidence that it is causal.”

Researchers speculate that the increase in mortality could result from factors such as sleep fragmentation — multiple awakenings throughout the night — a malfunction of the immune system, the body’s response to light and dark, fatigue, depression or undiagnosed underlying disorders, such as apnea or heart disease, among others, or a combination. (It’s also important to distinguish long sleep from a condition known as idiopathic hypersomnia, characterized by extreme tiredness, or sleepiness, during the day.)

Youngstedt theorizes that the effects of spending extra hours in bed may be no different from those of sedentary behavior that takes place out-of-bed. The latter contributes to obesity, cardiovascular disease, high blood pressure, depression, diabetes and musculoskeletal disorders, among other things.

“These are people who spend an hour or two more in bed, and we know that people who spend that kind of extra time in front the TV, or just sitting around, are susceptible to obesity and heart disease and so on,” he says. “There’s some evidence that long sleepers engage in less physical activity because they have less time in the day. You might be less likely to exercise if you are busy with school, or work or family.”

This raises an interesting question: Should long sleepers try to reduce their sleep time? Thus far, research suggests the answer is no. A 2008 study that Youngstedt conducted found no differences in glucose tolerance and insulin sensitivity (predictors of diabetes) among a small group of long sleepers after reducing their time in bed by 90 minutes for eight weeks.

In a second study, still underway, Youngstedt is testing both long (8-9 hours) and average (6-7¼ hours) sleepers to see if a one-hour nightly reduction over 12 weeks will affect glucose tolerance, inflammation markers in the blood, sleepiness and cognitive function. To be sure, while these studies focus on the short term, such changes can be an indication of future health problems.

For now, long sleepers don’t need to modify their habits.

"If you've routinely needed more than the average number of hours of sleep most of your life, and you don't have excessive daytime sleepiness or impaired daytime functioning, I would leave things alone," Shah says.

This is good news for someone like Kate, who really feels it when she can’t log all the sleep hours she needs.

“I have normal amounts of energy in the morning but by early evening, I’m tired and often feel like getting in bed by 7 p.m.,” she says. “Sometimes I fight it, while other times I just cave into my desire and go straight to La La Land.”