Tired? Join the club.

Feeling tired or fatigued is a common experience. Yet health-care providers often dismiss complaints about tiredness - both because the symptom is universal and because it can be challenging to evaluate medically, says Michael Grandner, director of the University of Arizona's Sleep & Health Research Program in Tucson.

And while tiredness is often temporary, treatable or nothing to worry about, experts say that tiredness that suddenly worsens or prevents you from doing what you want can be a sign of a health problem or sleep disorder.

"Sleep seems to be a canary in the coal mine, where it's sensitive to all these things going on in your body," Grandner says.

"So, when it starts changing, you want to ask, 'Well, what's going on?'"

Sleepiness, fatigue, tiredness: in conversation, people use the terms interchangeably. But medically, their definitions differ. Understanding the differences is an important first step toward tackling the problem - or figuring out if there is one.

Sleepiness is a need for sleep that makes it difficult to stay awake, even while driving, working or watching a movie, and even after ingesting caffeine.

Fatigue, on the other hand, is a deeper sort of an inability, either physical or mental, to do what you want to do, such as get to the grocery store.

Somewhere in the middle is tiredness, a desire to rest that is less debilitating than fatigue and less dramatic than sleepiness. You can still be productive while tired.

Whatever you call it, it's common. In a 2014 survey by the nonprofit National Sleep Foundation, 45 percent of adults said they had been affected by poor sleep or not enough sleep in the previous week.

As many as 20 percent of people report excessive sleepiness on a regular basis. And, a National Safety Council survey reported in 2017 that 76 percent of people felt tired at work.

If you're bothered by how tired you feel, there might be some simple explanations, including the most basic: not enough sleep. A third of Americans don't get the recommended seven or more hours a night, according to the Centers for Disease Control and Prevention. And because needs vary widely, even seven hours isn't enough for many people.

"If you're routinely getting five or six hours of sleep and you're feeling tired," Grandner says, "that's an easy thing to check off the list in terms of figuring out what the problem is."

Sleep deprivation is not just a nuisance. It raises the risk for car accidents and has been linked with health concerns such as Type 2 diabetes, cardiovascular disease and depression.

Lack of sleep can also affect mood and relationships in ways that even caffeine can't remedy, says Nathaniel Watson, director of the Harborview Sleep Clinic at the University of Washington in Seattle. "There is no substitute for sleep," he says.

Beware the temptation to lie down exactly seven hours before your alarm is set to go off. Nobody sleeps 100 percent of the time that they're in bed, Watson says, so it might take eight hours of pillow time to get seven hours of sleep.

The physiology of sleep might also be getting in your way, if only temporarily. A phenomenon called sleep inertia, for example, is what helps you fall back asleep after ordinary night wakings, which typically happen multiple times a night, Grandner says.

But sleep inertia will also make it tough to get up in the morning if the alarm rings during a deep stage of sleep. That grogginess should wear off within half an hour of pushing through it.

Also normal are occasional rough nights because of stress or sleep interruptions. And even if you get a good night's rest, you may experience a mid-afternoon bout of sleepiness as a result of ordinary circadian rhythms.

Age is something else to keep in mind, though the evidence there is somewhat counterintuitive. Studies show that, as people get older, sleep patterns tend to change in predictable ways. It may start taking longer to fall asleep. You may wake up more often and spend more time awake in the night. And bedtimes and mornings may shift earlier. Menopause is another common cause of interrupted sleep.

But sleep satisfaction doesn't necessarily drop with age. Studies by Grandner and others have found that complaints about sleep and tiredness actually decline with age after a peak in early adulthood. In other words, you should not blame aging if you find yourself struggling with tiredness.

"Aging is associated with sleep that is a little shallower and a little more broken up, but not less satisfying," Grandner says. "If you're an older person and you're really unhappy with your sleep, that's actually an issue."

For people of any age, if tiredness is making it hard for you to get through most days or otherwise getting in your way, experts suggest visiting a primary-care clinic first to be evaluated for common causes of fatigue or tiredness, including depression, autoimmune diseases, vitamin levels and thyroid issues.

One warning: The appointment might be frustrating. Many doctors lack training in sleep medicine, Watson says. Primary-care physicians don't routinely ask patients about sleep, Grandner adds.

They also often miss the signs of insomnia or they suggest ineffective treatments for it, a 2017 study found. Insomnia affects up to 15 percent of adults and, Grandner says, studies show that behavioral therapies work better than medication.

A friend of mine, a parent of a young child, told me that her doctor laughed at her when she mentioned she was tired all the time, as though that was a given at her stage in life.

Anecdotally, though, doctors' visits can turn up all sorts of conditions. Friends have told me about tiredness that led to diagnoses of iron deficiency, fibromyalgia, celiac disease, encephalitis and more.

If nothing turns up in the regular clinic, it's worth seeing a sleep specialist, whose evaluation is likely to include screening for sleep apnea. The disorder, which causes people to periodically stop breathing in their sleep, affects up to 10 percent of adults - with rates higher for people who are overweight. Most don't know they have it. About 85 percent of people who have sleep apnea are undiagnosed and untreated, Watson says.

Bottom line, experts say: Being tired is worth paying attention to. The good news is that causes are often treatable.

"If you're feeling sleepy and it's interfering with your life, you shouldn't just think this is normal kind of a thing," Watson says. "We need to realize that if we prioritize sleep, we become the best version of ourselves."

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This article was originally published by The Washington Post.