Caffeine: The Drug that Keeps us Alert by Day and Awake by Night

Caffeine is the most widely used stimulant in the world with many other attributes, some good and some bad. The good news is that it is not considered a drug of abuse in humans, but studies have shown it can be harmful and addictive in animals.

We know this because of the vast number of animal studies that examined caffeine and its effects with the intention of understanding its safety profile in humans.

At normal human doses, caffeine does not cause the happy hormone, dopamine, to be released by the part of the brain that reinforces addiction (the reward center). However, glucose is used up faster in the brain at high levels of caffeine consumption. This is reflected in some of the side effects experienced such as jitteriness or the ‘shakes’, anxiety, restlessness, and elevated pulse.  

Much lower doses of caffeine can affect the sleep-wake cycle and produce involuntary psychomotor symptoms such as tremors or twitches. Indeed, shakiness and eye twitching can be a common occurrence for many coffee drinkers — and tea drinkers too for that matter.

How much caffeine is in coffee? 

This seemingly simple question is surprisingly difficult to answer accurately. That’s because there are a bunch of other variables at play. For example, some roasts have more caffeine than others. Caffeine levels can fluctuate depending on factors such as the type of coffee and the extraction methods used.

Generally, caffeine content can range from 71-220 mg per cup of coffee (about 150mL). For adults who consume caffeine, the average daily intake is between 170-300 mg. 

Here are a few other beverages for context:

  • Tea – 32-42 mg/150 mL
  • Cola drinks – 32-70 mg/330 mL
  • Cocoa beverages – 4 mg/150 mL

How long does caffeine last?

The rate that caffeine is cleared from the body varies widely from individual to individual, based on genetics and the environment.

Smoking and oral contraceptives can speed up caffeine’s clearance from the body whereas heart failure, cirrhosis of the liver, and fluid in the lungs can slow its excretion.  The half-life of caffeine is about three to seven hours. That means that after about three to seven hours, the level of caffeine in the bloodstream is about half of the amount ingested. 

How does caffeine affect your sleep? 

One of the major reasons people drink caffeinated beverages is because they are sleepy and often the cause of this sleepiness is simply that they are sleep deprived. 

Whether that sleep deprivation is caused by a sleep disorder like insomnia or is simply due to a lack of sufficient time for adequate sleep, the results are the same. 

Often, drowsy individuals seek coffee or other sources of caffeine to feel more alert.

In many cases, this can create a vicious cycle where one consumes caffeine to stay awake, which makes falling asleep and staying asleep more difficult, causing even more caffeine consumption. Studies show that caffeine lowers the duration of sleep, causes problems falling asleep, increases awakenings, and increases daytime sleepiness.

The circadian rhythm can be adversely affected by caffeine, usually by delaying the body’s clock that manages the sleep-wake cycle and possibly affecting melatonin release. 

Several sleep parameters are negatively affected by caffeine intake. The researchers found that caffeine increases the amount of time it takes to fall asleep, decreases the duration of NREM sleep, and increases the number of nighttime awakenings. 

Taken together, caffeine intake is associated with a decrease in total sleep time.

Those with preexisting insomnia may have even more sleep problems with caffeine ingestion. The role of tolerance to caffeine and its effects on sleep has not been well studied. 

Some studies suggest that both sensitivity to caffeine and its metabolism are key factors in whether sleep is affected. Poor sleepers metabolize caffeine slower which can also explain the night-to-night variability in sleep onset and quality.

It appears that heavy caffeine drinkers are less prone to sleep disturbances than light drinkers. The accumulation of caffeine and its metabolites in the body and individual susceptibility to those effects are factors in the development of tolerance and how it affects sleep.

Since individuals metabolize caffeine at different rates, the time of day we consume caffeine can affect our sleep quality. Late afternoon and early evening intake are times when caffeine should be avoided.

Can you become tolerant to caffeine?

The term tolerance means an acquired change in responsiveness to a drug after repeated exposure to the drug. A tolerance for caffeine can certainly be developed.

A caffeine tolerance translates to a need for a higher dose to achieve the same effects of alertness. Coffee drinkers are well aware of how a regular cup of joe in the morning can turn into several cups of coffee over time. 

However, tolerance to caffeine that might escalate the intake is only present at low to moderate doses. This attribute explains people needing their first-morning cup, ASAP. At higher doses, the annoying physical side effects are increased and help prevent an individual from abusing or becoming tolerant to high levels.

Despite its lack of abuse potential, withdrawal from caffeine use can occur even at low levels. Headache and fatigue are the most common withdrawal symptoms. If caffeine is the source of sleeplessness, it is best to taper the level over the course of a few days to limit rebound headaches.

Withdrawal from caffeine can affect your sleep

Caffeine withdrawal symptoms begin 12-24 hours after you stop consuming coffee and are at their highest at 20-48 hours after. They can last a week. The withdrawal symptoms can occur even after low daily doses of 130 mg of caffeine.

The results of caffeine withdrawals manifest in the following symptoms. It’s worth noting, the symptoms are worse the first couple of days after quitting coffee.

  • Headaches
  • Fatigue
  • Drowsiness
  • Weakness
  • Irritability
  • Impaired concentration
  • Work problems
  • Depression
  • Anxiety
  • Increased muscle tension
  • Tremor, nausea, and vomiting (in extreme cases)

Withdrawal symptoms are based on the quantity of caffeine that you ingest. It can happen even at low doses of 130 mg up to over 2000mg per day. Interestingly, headaches before and after surgery are more common in people who ingest caffeine on a regular basis. 

How to take care of withdrawal symptoms

Withdrawal of caffeine can be treated with smaller amounts of caffeine. Withdrawal symptoms are linked to the psychological benefits related to the stimulant.

People who are heavy consumers of coffee, when subjected to decaffeinated coffee in lieu of their regular coffee regimine, will often feel the effects of headache and fatigue. When ‘non-caffeine’ people are subjected to caffeine, they experience anxiety and dysphoria in stark contrast to people who like caffeine. 

Alertness and uplifted mood are the common effects for people who are regular caffeine drinkers.

The safest and least painful way to reduce withdrawal is to taper your caffeine intake. You can do this by mixing decaffeinated coffee or tea with the regular in increasing amounts until you are drinking all decaffeinated.

Are neurological conditions affected by caffeine?


A retrospective study found migraine headaches responded well to medications that contained caffeine among women, menstruating or non-menstruating. Acetaminophen-aspirin-caffeine (AAC) treatment yielded improvements in migraine symptoms such as nausea and eye pain with bright lights and noises.

Parkinson’s disease

Even though the causes of Parkinson's Disease remain largely unknown, a few studies have assessed the risk of developing Parkinson’s disease with coffee consumption.  

According to a meta-analysis, coffee drinkers have a significantly reduced risk of Parkinson's disease. This may translate to other neuroprotective benefits of caffeine in other brain-based problems.


Caffeine acts on the brain as a stimulant and its effects on seizures are complex. Sudden and high levels of caffeine intake may induce seizures whereas chronic exposure may have an opposite effect. 

In a recent study, no difference was found between caffeine intake 24 hours before a seizure compared to consumption on a seizure-free day. The conclusion was that caffeine does not induce seizures.


Hemorrhagic stroke caused by a burst blood vessel can be precipitated by caffeine intake. Being a central nervous system stimulant, caffeine can raise both heart rate and blood pressure, which can elevate the risk of stroke if one is predisposed.

Caffeine and sleep: The bottom line 

According to a comprehensive review of the current research, caffeine can delay the onset of sleep, make you wake up more often, and possibly sleep less. These sleep issues can cause daytime sleepiness and create a cycle of needing caffeine to get through the day. 

The amount of caffeine and the timing were established factors influencing sleep. The sleep of older adults appears to be more sensitive to caffeine over younger adults. There are significant individual differences based on age and genetics. 

The take-home message is that caffeine at low to moderate levels in the morning are not associated with significant adverse outcomes regarding health. But if you are having trouble sleeping, removing coffee or tea from the equation might be your best bet. And don’t try to quit cold turkey — better to ease into it. 


  1. Burke TM, Markwald RR, McHill AW, Chinoy ED, Snider JA, Bessman SC, et al. Effects of caffeine on the human circadian clock in vivo and in vitro. Sci Transl Med. 2015 Sep 16. 7 (305):305ra146.
  2. Ruijter J, De Ruiter MB, Snel J. The effects of caffeine on visual selective attention to color: an ERP study. Psychophysiology. 2000 Jul. 37(4):427-39. 
  3. Quinlan PT, Lane J, Moore KL, Aspen J, Rycroft JA, O'Brien DC. The acute physiological and mood effects of tea and coffee: the role of caffeine level. Pharmacol Biochem Behav. 2000 May. 66(1):19-28.
  4. Strain EC, Mumford GK, Silverman K, Griffiths RR. Caffeine dependence syndrome. Evidence from case histories and experimental evaluations. JAMA. 1994 Oct 5. 272(13):1043-8.
  5. Chaudhary, N. S., Grandner, M. A., Jackson, N. J., & Chakravorty, S. (2016). Caffeine consumption, insomnia, and sleep duration: Results from a nationally representative sample. Nutrition (Burbank, Los Angeles County, Calif.), 32(11-12), 1193–1199. doi:10.1016/j.nut.2016.04.005
  6. Strain EC, Mumford GK, Silverman K, Griffiths RR. Caffeine dependence syndrome. Evidence from case histories and experimental evaluations. JAMA. 1994 Oct 5. 272(13):1043-8.
  7. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition. Washington, DC: American Psychiatric Association; 2013. 503-509.

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