We’ve all fought that 3pm slump. You struggle to keep your motivation, and the urge to grab a cup of coffee is the strongest force in the world. But how do you know if that feeling is just sleepiness or something more? Before you reach for that third cup of joe, you should know that it may be the wrong thing to do.

To find out more we spoke with caffeine expert and Food Science researcher Danielle Robertson-Rath of GreenEyedGuide Research and Consulting. Danielle holds her Masters in Food Chemistry from UC Davis, and recently published her second book, appropriately titled How to Get Sh*t Done When You Feel Like Sh*t.

The Difference Between Tired and Fatigued

If you’re feeling like your batteries are low in the afternoon, one of the first questions to tackle is whether you’re feeling tired or whether you’re feeling fatigue. Though they’re often used interchangeably, according to Rath’s book they’re not the same and shouldn’t be treated the same way. Fatigue is defined as “weariness from bodily or mental exertion” and tiredness or sleepiness is “the tendency to fall asleep.”

So how can you tell which one you’re feeling? The key is in the way they manifest themselves in your body.


“There’s a biological process called the sleep-dependent process. In this process, the pressure or impulse to sleep builds when we don’t get enough sleep or when we’ve been awake for an extended period of time.”


“Fatigue is less predictable than that. Fatigue can be physical (like after a tough workout), psychological (like after a mentally exhausting conversation with your boss), or both (like a physically and mentally exhausting day fighting fires or chasing your toddler around the house).”

If you regularly get less than the recommended number of hours of sleep then the cause of your energy crash is probably pretty clear.

Some other, less obvious causes of sleepiness can include

  • Poor diet
  • Eye strain
  • Dehydration

The cause of fatigue, on the other hand, may not be quite so obvious:

“When we talk about the fatigue of machines or equipment, it’s the tendency of that material to start to fail because it’s been worn down from excessive strain or prolonged use without repair. We can think of fatigue in people the same way.

“We all handle stress differently, so different situations might be more mentally or physically exhausting to you than someone else. You might feel fatigued (either mentally, physically, or both) because you’ve been pushing yourself to do more and more without having enough time to rest, recover, or readjust.” 

Will Caffeine Save Me From Fatigue?

If you’re stuck in the office, still hours away from blissful slumber, you might not care about the difference between tired and fatigued. The answer is just more coffee either way, right?

Hold on. Another dose of caffeine may actually be counterproductive.

“Caffeine is like alcohol in a lot of ways – there’s a proper time, a proper place, and a proper amount. If you’re awake but mentally fatigued, meaning stressed out or overwhelmed, caffeine might not be the best solution. Caffeine can increase your anxiety instead of helping you focus.”

There’s a little more grey area when you’re talking about a simple lack of sleep. Recent studies have shown that a dosage of 200mg of caffeine may be enough to offset one hour of sleep loss in participants’ performance in cognitive tests. (To see the importance of a good night’s sleep, our Coffee Calculator shows just how many cups you’d need to offset your own sleep deficit.)

That said, this shouldn’t be taken as a green light to go chug your favorite energy drink all day.

If you’re feeling sleepy, caffeine can definitely help but it could also backfire. If you drink too much caffeine too late in the day, it could make it harder for you to get to sleep. Then you wake up tired and need more caffeine. It’s an awful cycle, but it’s totally preventable if you know how to gauge your afternoon fatigue.”

Part of the issue lies with a chemical called Adenosine – a neurotransmitter that can cause feelings of sleepiness. Adenosine naturally builds in your body during wakeful hours and, as it binds with your brain’s receptors, your body adjusts accordingly. However, caffeine can also bind with adenosine receptors, preventing your body’s response and allowing adenosine to build in your bloodstream.

“The body is smart and can adapt like the Borg in Star Trek. I think of adenosine like the automatic filter on your phone that dims the lights of your screen, helping your mind realize it’s time for bed. Caffeine blocks adenosine from reaching its receptors and sending us those ‘You are getting sleepy/It’s time to rest’ signals. Eventually, the body makes even more adenosine to compensate. That means eventually we need even more caffeine to feel the same level of alertness or stimulation.”

How About a Power Nap?

Studies out of NASA’s Ames Research Center have shown that “[n]aps can maintain or improve subsequent performance, psychological and subjective alertness, and mood.” In fact, pilots within the study showed performance improvements of up to 34% in simulation tests.

But before you snag a snooze in the supply closet, you should know that a nap’s effects depend heavily on the timing.

“A good nap will help with sleepiness but not always with fatigue. And in some cases, a nap could be the worst thing you can do depending on the ‘cognitive load’ or the intensity of the work you have to do immediately after you wake up from your nap.”

That same NASA research also discovered that– depending on the duration and timing of the nap– waking in the middle of a sleep cycle could cause disorientation and sleepiness that lasted up to half an hour after waking.

What to Do About Fatigue

So what can we do when we hit that afternoon slump? The answer depends on where you fall on what Rath calls the “5 Levels of Fatigue.”

“You’ll never truly beat fatigue, but you can learn to see it as a tool instead of the enemy. Fatigue can be a very powerful signal if you know how to read it and how to respond.”

At the lower levels, which may be akin to being tired or sleepy, there are a number of possible solutions.

There are quite a few things you can do to feel more energized, more alert, and more engaged – with and without caffeine. Some of the actions I recommend are to get up, move around, and get some water. Dehydration and boredom can make you feel sleepy, so there are plenty of things you can do before having to brew a cup of coffee or crack open that energy drink.”

So the coffee may actually help here?

“In my book, I put a lot of focus on the different kinds of coffee, tea, or energy drinks you can drink and the different amounts of caffeine you can drink based on your ‘Level of Fatigue’. […] The mistake I see a lot of other authors and health experts make is they provide one-size-fits-all recommendations. For your health (both mental and physical health), you need a proportionate response.”

And what if you find yourself stuck in one of the higher levels of fatigue during work hours?

“This is a tough question to answer because like many of my clients, fans, and readers, I hate letting people down. I would rather push myself to my limits and maybe beyond that before asking for help or admitting I can’t do something I promised I would do. It’s always a tough situation, which is why I feel so strongly that my 5 Levels of Fatigue system can help people who often find themselves in this crux. If you work the system for every Level of Fatigue, it’s a lot easier to handle those heartbreaking moments when you feel you’re reaching your absolute limit.”

How to Stop Feeling Tired

In the end, there’s just no substitute for a good night’s sleep. But with 1 in 3 adults routinely failing to get the recommended amount of sleep, chances are you’re going to need a little help during your day.

If you’re feeling a little sleepy, you might try taking a break, try walking around, or grabbing a glass of water. A well-timed cup of coffee may help, but be mindful of when you drink it so as not to disrupt your body’s sleep cycle.

To learn more about the 5 Levels of Fatigue, and how to stay productive during each, check out Danielle’s new book How to Get Sh*t Done When You Feel Like Sh*t.

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