A Brief Guide to the Reasons You’re Always Tired

A comprehensive look at the causes of fatigue

A young woman yawning at her desk while looking at her computer.

FFeeling tired all the time is incredibly common. Dr. Sharon Bergquist, an assistant professor of medicine at Emory University School of Medicine, says that about a quarter to a third of primary care visits are due to fatigue. Feeling tired isn’t an automatic reason for concern — some level of fatigue is normal. “I think people expect themselves to do too much, so they’re just fatigued because they’re mentally or physically exhausted,” she says.

For some people, a few good nights of sleep will do the trick, but for others, fatigue can become an ongoing problem that interferes with everyday life. If tiredness doesn’t resolve even after adjusting activity levels or sleep routine, Bergquist says it’s important to talk to a doctor, who can help determine whether seemingly constant tiredness is due to lifestyle factors or a more serious medical condition.

Here are a few of the most common causes for chronic tiredness.

Poor diet

The body converts food into energy, which is why a healthy, balanced diet is essential for well-being. If you don’t get the right nutrients, the body won’t have the energy needed to function.

“A diet that isn’t dense in phytonutrients (chemical compounds produced by plants) like fruits and vegetables and is inflammatory is literally like putting cheap gas in an expensive sports car. It won’t run properly,” says Dr. Joseph Feuerstein, director of integrative medicine at Stamford Health in Connecticut. “Poor nutrition is like poor fuel, which results in poor performance.”

If you’re feeling regularly tired, start by examining what you eat. If your diet leans more into daily Pop-Tarts and McDonalds than fresh fruits and vegetables, that’s probably a factor for lower alertness.

“To perform well, you need to have phytonutrients as a dense part of your diet, along with lean proteins and judicious use of whole grains and lots of different healthy oils like nuts and seed oils,” says Feuerstein.

Whether you’re not consuming enough nutrients in your diet or your body isn’t absorbing them for some reason, vitamin and mineral deficiencies can cause exhaustion. For example, Bergquist says people who are low in vitamin B12, which helps the brain and nervous system function properly, often feel weak and fatigued. Typically, this is because B12 helps the body make red blood cells, which carry oxygen to your body.

Feuerstein says low vitamin D can cause people to feel listless and is often cited as a potential reason for fatigue. But since almost everyone in the United States experiences a deficiency (especially during the colder, darker winter months), it’s often difficult to know if fatigue is truly caused by low vitamin D levels.

Depression or anxiety

Mental illnesses, like depression and anxiety, can cause emotional symptoms like sadness and worry, but Dr. David Spiegel, a psychiatrist and director of the Center on Stress and Health at Stanford University, says they also affect the body.

“People who are depressed tend to have lower energy, which can be caused by sleep disruption, lack of physical activity, or just general trouble getting yourself together to do the things you need to do,” he says. “People with anxiety spend a lot of energy worrying about doing the wrong thing — so pile that on to the actual energy it takes to do what you want to do, and you’re expending a lot of energy.”

Chronic stress, which is associated with chronically elevated cortisol, can also lead to fatigue. “Typically, acute stress gets your blood pressure and heart rate up, so you feel more aroused,” says Spiegel. “But with chronic stress, you can feel burned out because you’re secreting more cortisol, so you deplete your reserves after a while.”

Not enough exercise

It may seem counterintuitive that burning energy can wake your body up, but Feuerstein says habitual movement is important for feeling energized. That’s because regular, aerobic exercise — Feuerstein encourages patients to aim for 30 minutes a day, most days of the week — benefits heart and lung function, making tiring or demanding activities easier.

“Exercise is no different than tuning up and looking after your car,” he says. “The human body needs to move regularly, and it will only function properly if it is.”

Too much exercise

On the other hand, exercising too often or too hard can deplete energy levels and cause fatigue. “It’s good to be physically active, but there comes a point when your body just can’t keep up,” says Spiegel. “You deplete your immediate supply of glucose and other nutrients that feed your muscles and give you energy, and then you start producing more lactic acid, which causes your muscles to feel sore.”

If you’re feeling exhausted — whether from a demanding schedule or because you’ve been working out a lot — focus on resting instead of placing more demands on the body.

In some cases, feeling tired all the time can be a sign there’s something else going on.

A thyroid problem

Once Feuerstein rules out basic lifestyle causes like lack of exercise and poor nutrition, he often looks into his patients’ thyroid function. Because your thyroid produces hormones that energize cells and regulate bodily functions like metabolism and blood pressure, fatigue is one of the most prominent symptoms of an under-functioning thyroid. Other common symptoms can include unexplained weight gain, hair loss and brittle nails, muscle aches, and sensitivity to cold.

While hypothyroidism (an underactive thyroid) is a common cause for feeling tired all the time, Feuerstein says some people experience fatigue with subclinical hypothyroidism, a milder and earlier form of true hypothyroidism where the body is struggling to produce enough thyroid hormones.


Many over-the-counter and prescription medications have side effects of fatigue or sleepiness. For example, Bergquist says she commonly sees people who don’t make the connection between their daily allergy pill or sleep aid and chronic fatigue. “You might sleep better with a sleep aid, but some of these over-the-counter pills can have residual grogginess that lingers the next morning,” she says.

Spiegel says anti-anxiety medications like benzodiazepines can lead to fatigue, and in some people, they can compound feelings of tiredness by causing depression. Medications like Xanax inhibit activation and excitement — so you may feel more mellow, but you could also get sleepy.

Alcohol and marijuana can have a similar effect, according to Spiegel. You may get an initial buzz, but in many cases, you end up getting more tired later on. Fatigue is more likely to increase with more regular substance use, and especially with abuse.

Lack of sleep

Feuerstein recommends adults aim for between six and ten hours of sleep a night — the National Sleep Foundation suggests between seven and nine hours — but there’s not necessarily a hard-and-fast rule about how many hours of sleep a person needs. The key is getting restful sleep. If you wake up frequently at night and can’t fall asleep relatively quickly, you’re more likely to feel tired.

Because the body repairs and heals itself when you’re asleep, long-term lack of sleep can be even more detrimental, leading to chronic fatigue or more concerning health problems. “Recent studies show cerebrospinal fluid rushes through the brain and body when we sleep, essentially cleaning things out and trying to repair,” says Feuerstein. “Sleep is a third of your life, so if you don’t do it properly, you’re kind of getting a failing grade on one of the most important things for your body.”

If you’re getting ample sleep but still not waking up feeling rested, you might have a sleep disorder like sleep apnea. While classic sleep apnea typically presents with severe snoring or lapses in breathing, for some people, it can be more subtle — so subtle a sleep disorder might not even be on your radar. Dr. Jenny Kim, a pulmonologist at Summit Medical Group in Livingston, NJ, says people with mild or moderate sleep apnea often wake up feeling unrefreshed no matter how many hours they slept, or they might feel drowsy or experience difficulty concentrating later on in the day. Waking up with a headache is another common indicator.

Kim says a simple screening test with a sleep specialist can diagnose even mild or moderate forms of sleep apnea, which is fairly easy to treat with a device that lowers the jaw to create extra space in the back of your throat.


Iron is an important part of red blood cell production, and red blood cells have the important job of carrying oxygen throughout the body. If you’re low in iron, then you’re probably going to feel weak and tired.

Bergquist says anemia can stem from extra blood loss due to heavy or irregular menstrual cycles or lack of iron in your diet (sometimes, people who don’t eat red meat have lower iron). But other, more serious issues can cause anemia, such as bleeding through the gut due to colon cancer or celiac disease.


Before reaching for a second cup of coffee during a mid-afternoon slump, think about water intake. Ample hydration is just as important for energy as nutrition. “Even losing one or two percent of your body weight in water can affect how you feel, from how clearly your brain thinks to how quickly your muscles fatigue,” says Bergquist.

Water carries nutrients to the cells and removes nutrients from the body, so if you don’t have enough, it impairs how every cell in your body can function. But don’t wait until you feel thirsty to up your water intake; Bergqust says you’re probably already dehydrated by that time.

Historically, doctors have recommended people calculate their ideal water intake according to their weight, but Bergquist says that it’s better to aim for lighter-colored urine rather than a certain number of daily cups. “How much water you need varies tremendously based on things like the altitude where you live, humidity, and the food you eat,” she says. “Looking at your urine to make sure it’s clear is the most predictable way to make sure you’re hydrated.”

Blood sugar problems

Bergquist says insulin resistance — which means the body can’t effectively take blood sugar into its cells for energy — is becoming increasingly common. So even if someone doesn’t have diabetes, they can experience blood sugar issues that might make blood sugar spike then crash after eating, which can cause feelings of tiredness, weakness, or shakiness.

Much of the time, lifestyle factors like eating a lot of refined, processed carbohydrates can impact people’s ability to process sugar properly, but you may also want to be screened for pre-diabetes or diabetes.

Underlying medical issues

Most fatigue is easily remedied with basic lifestyle changes, like getting more restful sleep, moving the body, or adjusting your diet to include more water and nutrients. But in some cases, feeling tired all the time can be a sign there’s something else going on.

If your tiredness persists after you alter your lifestyle or you’re feeling worse over time, it’s important to talk to a physician. In some cases, it could be a condition like Chronic Fatigue Syndrome.

“Some fatigue is short-lived and you can correct it, but if it becomes chronic and persists, that’s reason to go to the doctor to rule out worst-case scenarios,” Bergquist says.

Writer-mom hybrid. Health & psychology stories in NYT, WaPo, Allure, Real Simple, & more.

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