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Caffeine & Sleep: How Long Does Caffeine Keep You Awake?

Oct. 1, 2021 - Katie McCallum

Instead of counting sheep you're counting hours of sleep. Specifically, the number of hours you're not getting. Or, maybe you're counting the number of hours you might get if you fell asleep right now. Nope, still not asleep.

Was it that latte you had with lunch??

You know caffeine can keep you awake, and there are the obvious times to avoid it. (Right before bed, for instance...just in case...)

But, you needed that caffeinated pick-me-up to get through the rest of your busy, exhausting day. And, at the time, it felt like you needed it just as much as the sleep you're not getting now.

So, when it comes to that gray area between late morning and early evening, how late is too late to drink that afternoon coffee you so desperately need — or even just a soda or iced tea with dinner — without it affecting your sleep?

How does caffeine work?

Once consumed, caffeine is very quickly absorbed and distributed throughout your body, including to your brain. It's here that caffeine elicits its most classic effect — helping keep you alert and awake.

Caffeine accomplishes this by blocking sleep-promoting receptors in your brain called adenosine receptors. It's able to do so because — as far as molecular structures are concerned — caffeine looks very similar to one of the naturally occurring molecules in your body that typically binds to these receptors, called adenosine.

Adenosine plays many roles — including helping to regulate your sleep/wake cycle. By binding to adenosine receptors in your brain, adenosine activates the receptors. In turn, this triggers pathways that slow neural activity and increase feelings of sleepiness.

Adenosine levels in your brain fluctuate. Its levels are fairly low by the time you're fully awake, but they slowly build throughout the day. After several hours of being awake, adenosine levels increase to a point where they start the process of making you sleepy. Cue bedtime, and the cycle repeats the next day.

But, as mentioned, adenosine isn't the only molecule that can bind to these sleep-promoting receptors in your brain. Caffeine can, too. Except, when caffeine binds to these receptors, it doesn't activate them like adenosine. Instead, caffeine blocks them — preventing adenosine from binding and activating their sleep-promoting effects, keeping you awake and alert in the meantime.

Fortunately for your sleep hygiene, caffeine doesn't hang around in your body forever.

How long does caffeine last?

To measure how long a substance like caffeine lasts in your body, scientists use a term called "half-life." This is the time it takes for the starting amount of the substance to reduce by half.

According to the FDA, the half-life of caffeine is between four and six hours.

This means that up to six hours after drinking a caffeinated beverage, half of the caffeine you consumed is still present in your body — keeping you alert. And, if it's bedtime, keeping you from falling asleep.

How late is too late to drink caffeine?

We know how it works. We know about how long it can last. But, put together, what does this mean for someone who's trying to have some caffeine without it affecting his or her sleep?

Unfortunately, this is a question with a fairly murky answer.

The short-and-sweet version is that most experts recommend setting your caffeine cutoff for 2 or 3 p.m.

While there's plenty of research showing that caffeine disrupts sleep, only one study has examined how the timing of caffeine intake affects sleep. The study results showed that having caffeine even as early as six hours before bedtime can impact sleep, even if you don't actually notice the disruption.

However, it's also important to note that the study's 12 participants were given the maximum daily dose of caffeine (which is 400 mg, by the way). This might not be a realistic representation of the average adult's afternoon coffee consumption, but it may be a totally accurate representation of, say, a college student's afternoon caffeine consumption.

Still, this study generally supports sticking with the 2 to 3 p.m. caffeine cutoff window experts suggest — especially if you're someone who starts winding down for bed around 9 p.m.

Ultimately, though, there's probably no one-size-fits-all rule as to how late is too late when it comes to consuming caffeine.

The length of time your favorite caffeinated beverage might hang around in your system varies depending on:

  • It's caffeine content
  • How much caffeine you already had in your system
  • How effective your body is at metabolizing caffeine (which varies from person to person)

And while your favorite drink may not have 400 mg of caffeine in it like the participants in the study mentioned above, you may be surprised just how much caffeine is in some common beverages.

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