It seems to be a universal truth that we're all tired all the time.

The reason can be obvious. At least one third of U.S. adults report that they get less than the recommended amount of sleep, according to the CDC. This is sometimes out of our control, but a lack of quality sleep can also be self-imposed — whether we realize it or not.

And what about when you feel like you're always tired but can't figure out why?

"Fatigue is often the result of insufficient sleep or behaviors that lead to poor sleep," says Dr. Aarthi Ram, a neurologist specializing in sleep medicine at Houston Methodist. "But it can also sometimes be a sign of an underlying health issue that needs to be addressed or an undiagnosed sleep disorder, so it's important to consult your doctor if you're struggling with fatigue."

No matter the cause, it's an important issue to correct since daytime fatigue can impact your life in several ways — ranging from reduced productivity and performance at home or work to safety risks while driving or on the job.

Are you tired or are you just sleepy?

We often use the words "tired" and "sleepy" interchangeably, but the two are actually distinct. Tiredness is defined as being fatigued, whereas being sleepy simply means you're in need of sleep.

"Sleepiness is that draw to fall asleep, which builds the longer we're awake," says Dr. Ram. "If we go to sleep when we feel sleepy and get enough quality rest, we'll typically feel refreshed and energized the next day."

The cycle continues — unless something like a two-hour nap after work throws your sleep cycle off, pushing your usual 10 p.m. bedtime to the wee hours of the morning, even though your alarm clock is still set to go off bright and early. (Related: Is It Bad to Take Melatonin Every Night?)

"When you feel like you can't keep your eyes open after only getting four or five hours of sleep, that's sleepiness," says Dr. Ram. "The short-term solution is to get some sleep. If you frequently find yourself sleepy during the day, the long-term solution is often as simple as practicing better sleep hygiene."

Tips for maintaining good sleep hygiene include:

• Aiming to get seven to nine hours of sleep every night
• Going to bed and waking up around the same time every day, even on weekends
• Making sure your bedroom is cool, dark, quiet and free of distractions
• Putting devices and screens away at least 30 minutes before bed
• Avoiding the following right before bed: napping, drinking alcohol or caffeine, eating a heavy meal
  
  

(Related: Why Getting Too Much Sleep Makes You More Tired)

Getting quality sleep isn't always as simple as following this list, of course. For instance, a new parent who needs to mirror their baby's schedule won't be able to do much about their sleep hygiene for a while. In situations like this, Dr. Ram recommends trying to focus on the behaviors you can control and leveraging power-napping when the opportunity arises.

But if you're exhausted even though you feel like you're getting enough sleep? That may be tiredness and fatigue.

What is fatigue, and what are its most common causes?

Fatigue is when you're extremely tired — to the point where you don't feel like you have the energy to focus on tasks at hand or do the things you need or want to get done.

"When you're feeling run-down and exhausted by 6 p.m. every night and can't pinpoint a reason to explain why, that's when we start thinking about fatigue," says Dr. Ram.

The tricky part? Like sleepiness, fatigue can also be caused by poor sleep. But unlike sleepiness, simply getting quality sleep isn't enough to fix the problem.

"You may feel sleepy, but after sleeping you don't feel refreshed," adds Dr. Ram. "That's because, while poor sleep often contributes to fatigue, it's unlikely to be the only factor that needs to be addressed."

The lifestyle factors that cause fatigue include:

• Not getting enough quality sleep
• Poor diet
• Lack of physical activity
• Stress
• Alcohol or drug use
  
  

"There's interplay between these factors," says Dr. Ram. "An inactive lifestyle can lead to poor sleep, and poor sleep can lead to you feeling too tired to exercise. They compound upon one another to cause the mental and physical exhaustion we see with tiredness and fatigue."

To counteract this, start by improving your everyday habits — and not just aiming for more quality sleep. In addition to following the sleep hygiene tips above, this may also look like cleaning up your diet, making time for exercise, reducing alcohol intake and engaging in stress-relief practices, like meditation.

If you've made these changes and you're still always tired, it's time to talk to your doctor.

When the answer to "why am I so tired?" is more complicated

Fatigue is often the result of everyday lifestyle factors, but not always.

"When behavior modifications don't resolve the issue, we have to consider the other, less common causes of fatigue," says Dr. Ram. "These include everything from sleep disorders and medications you may be taking to the many health conditions of which fatigue is a symptom. Of which, some are more common than others."

For instance, obstructive sleep apnea is a very common condition that can cause daytime fatigue. It's diagnosed using a sleep study.

"A person with sleep apnea might not know they have it," says Dr. Ram. "It's not uncommon for someone who's tired all of the time and doesn't feel rested after adequate sleep to be struggling with this issue without knowing."

(Related: Snoring Isn't the Only Sleep Apnea Symptom)

Prescription medications — like benzodiazepines, beta blockers and opioids — and over-the-counter options, like certain antihistamines, can also cause fatigue. So can many medical conditions, ranging from vitamin deficiencies, weight issues, anxiety and depression to hormonal imbalances, autoimmune disorders, heart problems and more. (Related: The Signs You May Be Low on Vitamin D)

"With so many potential causes of fatigue, it's best to consult your doctor if you're frequently tired and can't pinpoint why," recommends Dr. Ram. "The problem could be as simple as a lifestyle behavior you hadn't yet considered, or it could be an underlying issue that needs to be diagnosed and addressed."

Your doctor will ask you questions about your sleep hygiene, lifestyle, personal and family health history and any other symptoms you're experiencing, information necessary to assess whether lifestyle modifications might help or if further work-ups, like blood tests or a sleep study, are needed.