Caffeine is a very popularly used agent. It is present in tea, coffee, dark chocolate, and even energy drinks. However, most people use caffeine in the form of tea or coffee to stay awake and alert, and for good reason as it is very effective in doing just that.

But how does caffeine prevent sleep? When should be our last cup of caffeine-containing beverage/food?

Caffeine prevents sleep by blocking the action of adenosine in our brain. It does so by blocking the adenosine receptors. Adenosine is the major trigger for our brain to induce sleep. Additionally, caffeine also delays sleep by altering the circadian rhythm.

Tea and coffee have many health benefits, but their caffeine content can severely hamper our sleep cycle. Caffeine-sensitive people and those who are light sleepers should be careful about when they have their last cup of tea or coffee as they may find it difficult to sleep at night. 

We will explore when our last cup of tea or coffee should be and how caffeine prevents sleep in more detail. But to understand that better, we’ll first see how we fall asleep in brief.

How Does Our Brain Know When It Is Time To Sleep?

Two important factors decide or let us know when we need to sleep:

1. The circadian rhythm
2. The level of adenosine in our brain.

Caffeine affects both of these processes.

The Role of Adenosine in the Brain

Adenosine is the degradation product of ATP and the major sleep trigger for the brain.

ATP is what gives us energy. 

You might remember ATP from your biology lecture in school— mitochondria are the powerhouse of the cells because it produces ATP. Food is broken into glucose, and our body produces ATP using glucose.

Our brain uses about 20% of the total body’s energy, as its energy requirement is very high. Once the ATP is utilised, adenosine is released as the metabolic product of ATP. 

Our brain perceives the presence of adenosine with the help of adenosine receptors. Adenosine binds with these receptors and passes the signal to the brain that too much work done; it is time to sleep now.

Once adenosine bind with its receptors, adenosine causes:

1. Inhibition of the wakefulness-promoting areas of the brain, and
2. Stimulation of the sleep-promoting areas of the brain.

So, with all the activities we do in the day, our brain uses more and more ATP to perform its normal function, and thus, more and more adenosine is produced as a result.

How much adenosine is produced determines how sleepy you feel at a given time. We fall asleep when the adenosine levels in our brains are extremely high. 

It is one of the metabolic wastes our brain clears up while we sleep. For this reason, the amount of adenosine is the least in the morning— given that you had a good sleep, and it increases in amount as the day progresses.

If we don’t get a good quantity and quality of sleep, adenosine is not cleared up properly. As a result, we feel sleepy and fatigued all day long. 

The Role Of Circadian Rhythm

Our circadian rhythm is the other thing that lets our brain know when it’s time to sleep. It’s our natural clock— which functions independently but uses sunlight as a guide. 

With the help of melatonin, it knows when it is daytime and nighttime. And this is how it wakes you up and makes you fall asleep in the morning and at night, respectively.

So, How Does Caffeine Prevent Sleep? 

The Effect Of Caffeine On Circadian Rhythm

As per some studies, caffeine pushes our circadian rhythm behind by 40 mins. That means sleep is pushed back by 40 minutes— which causes delayed sleep.

Forty minutes may not seem like a lot, but it is. 40 minutes of late sleep means 40 minutes of less sleep each night– considering most of us have to get up at the same time for work or school. That is a significant number because most of us today are not getting the recommended amount of sleep anyway.

The Effect Of Caffeine On Adenosine

Caffeine is structurally similar to adenosine. When we consume caffeine—coffee, tea, dark chocolate—it goes to the brain and binds with the adenosine receptor (since it is structurally similar).

When caffeine binds with the adenosine receptors, it makes the receptors unavailable for the adenosine. As a result, adenosine can’t trigger the brain for sleep, as it can’t bind with its receptor. 

In technical terms, this type of blocking is called competitive inhibition.

What Is Caffeine Crash?

With the help of caffeine, we can indeed prevent sleep. But the action of caffeine in our brain only delays sleep and does not eliminate it.

We may be drinking coffee and preventing sleep, but the brain is still functioning. That means adenosine keeps building up more and keeps accumulating.

Now, here’s the thing: anything we consume is metabolised in our body, including caffeine. With the help of an enzyme in our liver, caffeine is soon metabolised and removed from our bodies. 

Once the caffeine is metabolised and removed, the adenosine receptors are free again. And now adenosine can come and bind with them to trigger sleep. But by this time the build of adenosine is so great, that you can’t help but crash into sleep— as a result of the sudden wave of adenosine binding with the brain. This is called a caffeine crash. 

Drinking more caffeine is the only way to prevent caffeine crash, so you keep reaching for caffeine repeatedly throughout the day.

Caffeine Is An Addictive Substance

When you constantly drink caffeine, you become addicted to it. What we forget is that caffeine is not just a drink—but a psychoactive agent.

Therefore, relying on caffeine to stay awake only promotes caffeine addiction. While caffeine addiction may not be as severe as other typical drugs, caffeine withdrawal does cause some distressing symptoms (when the level of caffeine goes down in your body) like:

1. headache, 
2. palpitation, 
3. irritability, 
4. inability to focus, 
5. tiredness, 
6. nausea, and even 
7. muscle pain. 

These symptoms are relieved when you consume caffeine again— like what happens with typical drug addiction cases.

So, When Should Be Your Last Cup Of Coffee?

As mentioned above, with the help of enzymes, our body eliminates caffeine from our system.

The half-life is caffeine is approximately 5-6 hours on average. This means, 5 hours after you had the coffee, its effect will start wearing off. But this is the half-life— meaning, half of the caffeine is still in our system at this point, and the remaining half amount of caffeine is still capable of disturbing our sleep cycle.

We can only have good quality sleep when there is no caffeine in our system. 

Please note, that it takes about 1.5 to 10 hours for our body to eliminate caffeine from our system. Yes, that’s a huge range. This is because it depends on our body’s ability to metabolise caffeine. In some people, the caffeine metabolising enzymes are stronger. Caffeine in these people will be eliminated quickly. 

However, in some people, these enzymes are slow acting. In these groups of people, their bodies may take up to 10 hours to eliminate caffeine from their system. 

These are caffeine-sensitive people. 

Even small amounts of caffeine are enough to agitate these people. If they drink a cup of coffee in the afternoon, they will have difficulty falling asleep at night, as the caffeine is still present in their system by then, due to slower metabolism.

Therefore, deciding when should be your last cup of coffee will depend on your body’s independent capability to metabolise caffeine. But as a general rule, your last cup of coffee should be at least 8-10 hours prior to your scheduled bed time, as caffeine consumption even 6 hours prior to bedtime . can seriously disrupt your sleep even in non-sensitives, as per studies.

How Much Caffeine Can You Drink Per Day Safely?

Most people have normal tolerance to caffeine. Meaning, they won’t experience any negative effects (except sleep disturbance if taken later in the day) if they are consuming caffeine within the normal limits.

The recommended limit of caffeine by the FDA is 400 mg per day. For pregnant women, 200 mg per day is the upper limit. 

If you are pregnant, please be careful about your total caffeine intake, and stay within the ACOG recommended limits of 200 mg per day. An amount higher than 200 mg per day is associated with small babies, preterm deliveries and miscarriages as per some studies.

Also, please remember that even though 400 mg of caffeine is considered safe, it’s possible for a caffeine-sensitive individual to feel agitated with just 100-200 mg of caffeine.

It’s recommended to monitor your caffeine intake accordingly.

How To Know If You Are Consuming Too Much Caffeine?

You can know if you are consuming too much caffeine by observing how you feel and by noticing your sleeping pattern. If you are consuming too much caffeine then your sleep cycle will be affected. Period. You can look for some common signs to know if your sleep cycle is being affected. 

You are not getting enough sleep:

1. If you are having difficulty falling asleep, staying asleep, and difficulty falling asleep again after waking up at night,
2. The number of nighttime awakenings, and
3. If you are waking up feeling tired, fatigued, or have headaches— as a result, your focus and concentration are poorer than usual,
4. you need too much caffeine to stay awake and alert.

In addition to these, if you are caffeine sensitive, you may experience significant distress after consuming a very small amount of caffeine (say, half a cup of coffee or even less). You may notice some additional symptoms when you consume caffeine, like:

1. Anxiousness
2. Increased heart rate/ palpitations
3. Restlessness
4. Tremors
5. Headache
6. Insomnia
7. Inability to focus
8. Irritability

These are the common symptoms of too much caffeine intake and caffeine sensitivity. If you are experiencing these symptoms, you likely need to monitor your caffeine intake.

What If You Can Consume Caffeine And Still Sleep At Night?

Yes, this can be true too. You may be able to drink caffeine after dinner and still be able to fall asleep at night.

However, studies suggest that even in these cases, caffeine does affect your sleep. Caffeine may not stop you from falling asleep at the right time, but it will reduce the total duration of your deep sleep by 30%.

This means, when you’ll wake up the next morning, you won’t feel well-rested and you will need more coffee to get through the day.

So, it’s likely that people under this category (which is most of us) may incorrectly think that caffeine doesn’t affect their sleep when in reality, it does. 

You can experiment with this and see for yourself. You will be able to notice that if you have coffee in the evening, you don’t wake up that fresh the next morning. This creates a cycle of poor sleep leading to increased caffeine demand the following day. On the other hand, if you avoid caffeine past noon, you’ll wake up feeling more refreshed the next day and your requirement for caffeine will be reduced significantly. 

The Points To Keep In Mind About Caffeine And Sleep

Caffeine is not bad. In fact, tea and coffee have some really amazing health benefits. A cup or two of tea or coffee per day isn’t an issue. But consuming too much caffeine and consuming caffeine later in the day can disturb your sleep and lead to many health issues associated with lack of sleep.

It’s easy to overdo caffeine if you haven’t slept well at night. The lack of sleep- caffeine cycle can be never-ending. You can, however, deal with overdoing caffeine by carefully monitoring your habits and being mindful of your caffeine intake. 

Here are some of the things you should remember regarding caffeine and sleep:

• Caffeine is a psychoactive agent that you can get addicted to.
• While caffeine seems like a good way to prevent sleep, it might seriously hamper your sleep quality.
• The last cup of caffeine will depend on how quickly your body can remove caffeine from the system, but as a general rule 8-10 hours before your bedtime should be your last cup of coffee/caffeine.

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