The Relationship Between Caffeine and Sleep

Long Story Short

• Roughly 90 percent of adults worldwide consume caffeine on a daily basis, making it one of the most consumed stimulants in the world. (1)
• It takes about ten hours for caffeine to fully clear your system, which is why most experts recommend avoiding it at least eight hours before bed. (2)
• Despite this, 30 percent of adults drink some caffeine at or after 12:00 p.m., when it’s more likely to negatively impact their sleep. (3)
• Caffeine reduces total sleep time by 45 minutes, sleep efficiency by 7 percent, and causes you to take longer to fall asleep and fully wake up in the morning. (4)
• The Food and Drug Administration (FDA) recommends no more than 400 mg of caffeine per day, which is equivalent to about four cups of brewed coffee, ten cans of soda, or two energy-shot drinks. (5)
• The FDA recognizes that caffeine sensitivity levels vary from person to person.(5) If you think you’re experiencing symptoms of caffeine overconsumption, it’s a good idea to reflect on your caffeine intake and consider where realistic changes can be made to benefit your health and improve your sleep.

If you’ve ever started your day with a cup of strong coffee or packed energy shots in your travel bag, you’re among a solid crowd of caffeinated peers. While caffeine in general isn’t inherently bad, it can become problematic when you consume it too close to bedtime or in excess. So how can we make caffeine and sleep coexist? How much is too much? Are there alternatives for those of us who may be going overboard? We’re answering these questions and more. 

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately. 

How Does Caffeine Work?

Most of us know caffeine as the stimulant we rely on to get through a Monday morning meeting or to help us pull an all-night study sesh or road trip across state lines — but how does it work, exactly? Well, it has to do with how caffeine interacts with compounds in your brain. 

Alex Dimitriu, M.D., double board-certified in Psychiatry and Sleep Medicine, tells Sleepopolis that caffeine is a blocker of adenosine — a substance that relaxes your blood vessels and affects your heart rhythm. “Adenosine builds up in the brain as we get through the day, and when levels are higher, we begin to feel tired and sleepy. [Caffeine blocks adenosine], which is the reason it makes us feel awake and alert,” he says. 

Caffeine predominantly works as a stimulant in your central nervous system by blocking adenosine receptors and preventing its slowing effects on the mind. This mechanism also triggers the release of brain chemicals like dopamine and norepinephrine, which boosts mood, alertness, focus, and overall brain activity. (6) And if you’ve ever noticed that drinking too much morning coffee makes your heart race, that happens because of everything going on in your brain as well. 

As for how quickly it can start affecting you, caffeine is absorbed within 45 minutes after consumption and peaks in the blood anywhere from 15 minutes to 2 hours after ingestion. (7)(8)

Benefits of Caffeine

Look, there’s no denying it: in small doses, caffeine can give you a much-needed boost in concentration and energy. Productiveness and a sharp wit feel great, so why wouldn’t you drink a delicious beverage that gives you both?

Here are some of the known benefits of caffeine (in moderation):

• Energy: Research has proven the energizing effects of caffeine. (9) The reason you feel more alert and awake after a cup of coffee or an energy drink is because of how caffeine blocks adenosine activity in your brain.
• Improved Mood: While caffeine is pumping through your system, it can improve your mood. (10) You may have experienced this yourself, especially in the midst of a groggy morning or an afternoon food coma. Be careful, though. Too much caffeine can shift your good mood into anxious tension. (11)
• Better Concentration: Caffeine not only makes you feel like you can do everything twice as fast, but it can also improve your attention and concentration. (12) The bad news? Caffeine’s effects are temporary. That’s why you have to go back to the coffee pot or pop open another soda after lunch: You’re out of “gas.”

How Long Does Caffeine Last?

The focus-boosting effects of caffeine don’t last forever. It’s important to know how long caffeine is likely to affect you, so you can plan your intake accordingly as not to negatively impact your sleep. Experts often look at the half-life of substances, which is the time it takes for it to reduce to half of its initial amount. 

Dimitriu says caffeine has a half-life of approximately six hours, so you’re looking at 10-12 hours of caffeine remaining in your system. (5) He explains, “This means that every six hours, your body breaks down about half of the caffeine you consumed. Indeed this may be faster (not by much) in regular caffeine drinkers.” 

When you still have some caffeine lingering in your system, you might be able to fall asleep, but your rest may not be as restful as it would be otherwise. (13)

Additionally, Chester Wu, M.D., a sleep medicine physician in Houston, TX explains that everyone breaks down caffeine differently. Some factors that affect caffeine clearing include individual characteristics and environmental characteristics that influence caffeine metabolism, like: 

• Age (14)
• Body weight (14)
• Gender (15)
• Genetic factors (16)
• Health conditions (16)
• Medications (including birth control) (16)
• Whether you smoke (17)

You can follow the 10-12-hour rule of thumb, but you may need to do a little trial and error to find out how long it takes your body to kick out the last bit of caffeine.

How Does Caffeine Affect Your Sleep?

Caffeine negatively impacts sleep — especially when consumed too late in the day — because it disrupts the natural activity of adenosine in your brain, preventing you from feeling sleepy and relaxed when your body would normally be preparing for bedtime. (11)

This can make it harder to fall asleep, leaving you staring at the ceiling or caught up in an extended doom-scroll despite a comfy mattress. It may also shorten the amount of time you’re asleep through the night and decrease the quality of your much-needed deep sleep stages for rest and recovery. 

Still, according to Dimitriu, everyone is different, and our natural genetic and biological traits may impact things like anxiety, sleep quality, as well as our response to caffeine. For example, some people who struggle with anxiety (specifically, panic disorders) may find that they have a high sensitivity to caffeine and it affects them even in low doses. (18)

How Many Hours Before Bedtime Can I Have Caffeine?

Should you stop caffeinating after your morning buzz, or is an afternoon energy shot okay? It depends, but a good rule of thumb is no caffeine past noon for most.

Dimitriu notes that everyone gets sleepy in the afternoon, and if you’re sleep deprived as it is, it tends to be very notable around siesta time, or between 2 and 5 p.m. This can understandably have many of us reaching for a new pot of coffee in hopes of finding a small burst of afternoon energy to get us to bedtime. 

However, drinking caffeine this late in the day, and when you’re already yawning, is essentially too late, which is why Dimitriu recommends his patients stop drinking caffeine 2-3 hours before they typically feel that afternoon tiredness. 

Wu adds, “Because it’s hard to know how much caffeine you’re drinking as well as how you metabolize caffeine, a safe rule of thumb is to work back 10-12 hours from when you aim to go to bed. So, if 10 p.m. is your desired bedtime, you should last consume caffeine somewhere between 10 a.m. and noon.” (19) Other experts say to ditch all caffeinated products at least eight hours before bed. (20)

One review concluded that evening intake of caffeine, specifically more than 2 mg per kilogram of body weight (for instance, 128 mg of caffeine for a 63.3kg (140-lb) woman) at or after 5 p.m., significantly reduced the length and quality of sleep people achieved and increased how many times they awoke through the night. (21)

Overall, Dimitriu tells Sleepopolis he advises drinking as little caffeine as needed and slowly lowering your dose. “Having done this for years with patients, it’s amazing to see how well people do on far less caffeine than they drank before. It just takes time to adjust.”

Why Does Caffeine Make Me Tired?

There are a few reasons why some people are ready to bundle up in their cozy pajamas after consuming caffeine. One reason is that your body has a compensatory response — in other words, after its initial stimulating effect, caffeine can have a rebound effect during which adenosine builds up. Once the caffeine wears off, the high levels of adenosine get to work making you sleepier. (22)

Additionally, if someone frequently sips caffeinated beverages like they’re water, it’s possible to develop a tolerance. This means that you get used to the high caffeine dose and over time, its stimulant effects don’t work anymore. You may even experience tiredness as a form of withdrawal and tolerance. (22)(23)

Finally, drinking a lot of caffeine, or drinking it at irregular times (or late in the day) can interrupt your normal sleep patterns. (19) Ongoing sleep disruption or deprivation can make you feel more tired than normal even if you get a temporary boost in alertness from caffeine.

How Much Caffeine Is Safe To Drink?

The Food and Drug Administration (FDA) recommends a max of 400 mg of caffeine per day — that’s equivalent to about four cups of brewed coffee, 10 cans of soda, or two energy shot drinks. (5)

For reference, the average cup of coffee (like, an actual cup, not your 32-ounce to-go thermos) contains 40-150 mg of caffeine. (24) Multiply that by 3 cups per day and you’re looking at anywhere from 120-450 mg of caffeine. 

However, focusing on your caffeine timing can be much more helpful than fixating on the amount, says Wu, “as it’s hard to tell exactly how much caffeine you’re consuming and how much caffeine is right for you.”

Dimitriu says that, if you need a caffeine boost later in your day, it’s probably best to leave the venti (20oz) Starbucks brewed Pike Place coffee off the list. This contains a whopping 410 mg of caffeine, which is above the FDA’s recommended amount and therefore a lot for the average person. It’s important to understand how much caffeine is in popular beverages, as it can add up and some may even surprise you. See this guide for 45 beverages ranked by caffeine content.

And while 400 mg of caffeine is OK’d by the FDA, one 12-ounce can of Coke contains 33.5 mg of caffeine and a whopping 37 grams of added sugar, which isn’t great for our sleep or overall well-being. (25) It’s a good idea to check the nutrition facts panel to help you balance caffeine and added sugar intake from the various beverages you enjoy. 

Let’s break this down a little further into age groups. Below you’ll find a table of suggested caffeine limits based on how old you are. (26)

Suggested Caffeine Limit by Age Group

Age Group	Suggested Caffeine Limit
Children 4 – 6 years	45 mg per day
Children 7 – 9 years	62.5 mg per day
Children 10 – 12 years	85 mg per day
Children 13 years and older	2.5 mg per kg of body weight per day
Healthy adults	400 mg per day
Breastfeeding or pregnant people	300 mg per day

Some people may find a small cup of coffee or energy shot in the morning to be sufficient to fuel their day, but others require more. We all have different preferences, but the average daily consumption of caffeine in the US is about 135 mg. (7) 

Side Effects of Caffeine

Not everyone who drinks caffeine will experience side effects, of course. Caffeine sensitivity can vary from person to person, especially if you’re taking certain medications or have pre-existing medical conditions. However, some of the more commonly reported side effects of caffeine intake include: (27)

• Insomnia: Caffeine blocks adenosine receptors, which interrupts your natural tendencies to get sleepy and may make it harder for you to fall or stay asleep.
• Restlessness: By making your brain more active and alert, caffeine can make some people jittery or nervous and have a hard time relaxing. 
• Increased Heart Rate: Caffeine can make your heart beat faster because it causes your brain to release adrenaline.
• Tummy Troubles: If caffeine gives you heartburn or an upset stomach, that may be because of how it increases stomach acid.
• Dependence: You can eventually become dependent on caffeine if your body becomes used to drinking it all the time. If caffeine intake suddenly stops, you might feel withdrawal symptoms like headaches, tiredness, and irritability.
• Anxiousness: High doses of caffeine can worsen anxiety in some people because it increases stress hormones.
• Increased Blood Pressure: Caffeine may cause a temporary rise in blood pressure, which may be concerning for people who already have high blood pressure levels.
• Caffeine Crashes: After the stimulating effects of caffeine wear off, you might feel the effects of a “caffeine crash”, a drop in energy that switches to sluggishness. 
• Mood Swings: If you notice changes in your energy and ability to focus around caffeine consumption, this can be frustrating and might lead to mood swings. 
• Difficulty Concentrating: While caffeine can help you focus better at first, this wears off and may eventually make it harder to concentrate. 

Finally, while we often joke about “running on coffee”, there is such a thing as too much caffeine. The risk of caffeine overdose rises with higher product concentration. Just one teaspoon of pure powdered caffeine equals 28 cups of coffee, and a half cup of a highly concentrated liquid equals over 20 cups. (5)

Of course, most of us aren’t ingesting pure caffeine — but such toxic amounts can cause serious health issues, such as: (24)

• Nausea and vomiting
• High blood pressure following by low blood pressure
• Fever
• Rigid muscles
• Hallucinations
• Seizures
• Death

This makes it essential to take inventory of how much caffeine you’re ingesting, which may be from multiple sources, and whether it’s a healthy amount. (5) If you’re experiencing any of the symptoms above, it’s a good indication that you should check in on your caffeine habits. 

Alternatives To Caffeine 

Don’t worry — we’re not trying to take away your beloved coffee altogether. But if your caffeine intake has become habitual and is having negative effects on your restfulness (and ultimately, your ability to function at full capacity during the day), it’s time to look into alternatives to caffeine. 

Fortunately, there are plenty of options that may just do the trick. If you’re looking to replace coffee, soda, or energy drinks, these might suit your fancy: 

• Herbal teas like chamomile and peppermint 
• Decaf black and green teas
• Ginseng (28)
• Mushroom coffees, which blend coffee beans with medicinal mushrooms (29)
• Decaf sodas, like Sprite or Root Beer
• Soda water or seltzer water, which contain no sugar or caffeine
• Fruit-infused water 

Dimitriu says, “I personally enjoy green tea or yerba matte tea as smoother alternatives to caffeine. L-theanine in green tea has a nice calming effect (and is used as a supplement for anxiety and insomnia), but be sure not to drink these past noon either.” 

FAQs

The Last Word From Sleepopolis 

For many, caffeine fulfills an almost cultural role. You’ve been told by beloved coffee brands that no morning is complete without a steaming cup of coffee, and soda companies have worked hard to tell you that soda is the best drink when you need a cooling refreshment or just to have fun. There are energy shots at every check-out counter where you might be stopping on a long road trip. 

While caffeinated beverages are widely accessible and almost expected to be consumed, that doesn’t mean it’s always the best choice. Take a moment to reflect on your caffeine intake, note whether you’re experiencing signs of overconsumption (like poor sleep), and take the next step that makes sense for you. 

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Dimitriu, Alex. Personal interview. August 6, 2024.

Wu, Chester. Personal interview. June 22, 2023.