Regular caffeine consumption reduces sensitivity to caffeine. When caffeine intake is reduced, the body becomes oversensitive to adenosine. In response to this oversensitiveness, blood pressure drops dramatically, causing an excess of blood in the head (though not necessarily on the brain), leading to a caffeine withdrawal headache.
This headache, well known among coffee drinkers, usually lasts from one to five days, and can be alleviated with analgesics such as aspirin. It is also alleviated with caffeine intake (in fact several analgesics contain caffeine dosages). Even small amounts of caffeine (such as a green tea, compared to a full coffee) can do wonders to alleviate a withdrawal-induced headache.
The top 10 reported symptoms of caffeine withdrawal:
- Fatigue, lethargy
- Muscle stiffness, cramping
- Brain fog, Inability to focus
- Cold-like symptoms
Often, people who are reducing caffeine intake report being irritable, unable to work, nervous, restless, and feeling sleepy, as well as having a headache. In extreme cases, nausea and vomiting has also been reported. These are very real experiences , and despite recurring jokes, can cause problems with normal functioning.
Is Caffeine Withdrawal Real?
In short: Yes. Negative effects from quitting caffeine have been scientifically documented in clinical studies.
Of 49 symptom categories identified, the following 10 fulfilled validity criteria: headache, fatigue, decreased energy/activeness, decreased alertness, drowsiness, decreased contentedness, depressed mood, difficulty concentrating, irritability, and foggy/not clearheaded. In addition, flu-like symptoms, nausea/vomiting, and muscle pain/stiffness were judged likely to represent valid symptom categories. 
Additionally, caffeine withdrawal is recognized by psychiatrists as a real disorder.
Caffeine withdrawal is a recognized disorder and is listed in the DSM-5.
How Long Does Caffeine Withdrawal Last?
The answer to this will depend on your level of consumption and the level you reduce your caffeine consumption to.
In experimental studies, the incidence of headache was 50% and the incidence of clinically significant distress or functional impairment was 13%. Typically, onset of symptoms occurred 12-24 h after abstinence, with peak intensity at 20-51 h, and for a duration of 2-9 days. 
That means you’ll probably notice the start within 12-24 hours with the worst being the second day you after you quit. From there, it gradually gets better over the course of a week to a week and a half.
Your brain is miraculously resilient and adapts to life without caffeine in just 2 short weeks. The psychological habit of drinking caffeine can take 3 to 4 weeks to break, but can also be replaced with decaf coffee or another low-caffeine drink (eg. tea).
Reports of people having withdrawal symptoms months or years after quitting coffee or caffeine are more typically confused with general health maladies or other drugs.
Dealing with Caffeine Withdrawal
The severity of caffeine withdrawal symptoms vary with how extreme the restriction, and a gradual reduction can do wonders in avoiding symptoms. Simply starting by replacing one cup of coffee with a decaf coffee or tea will provide a much smaller dose of caffeine, allowing your receptors to re-acclimate to lower levels. Drinking coffee (decaf) or other warm beverage (tea) instead of regular coffee helps psychologically with the well established habit.
Timing large reductions in caffeine consumption is also a useful tool. Picking a time of rest and relaxation such as a weekend or vacation can lessen the burden of symptoms like brain fog and a lack of motivation.
Hydration is also key – while coffee is a diuretic it’s also mostly water, and cutting back on coffee may also inadvertently cut back on your fluid intake. Many symptoms of dehydration overlap with caffeine withdrawal including headaches, cramping, irritability and lethargy.
Without caffeine blocking your adenoseine receptors, your body’s built up levels of adenosine will lead to a lot of sleepiness. Get lots of rest! Scientists are still puzzled by why humans need sleep, aside from getting tired. According to some research from the NIH (National Institutes of Health) has indicated that sleep enables your brain to drain built-up chemical by products of working so hard (you little genius), which is good for you!
Advil or Tylenol can be an effective method of dealing with the coffee withdrawal headache. Other natural pain relievers such as running / exercise and even an orgasm can provide temporary relief by dilating blood vessels in the brain.
The best solution may not be totally ceasing caffeine consumption though. Coffee does have health benefits, as it contains over 1000 known compounds, with many associated with lowering blood pressure and improving cardiac function, as well as liver protection . Generally, the best coffees are grown at higher elevations will develop more healthy chlorogenic acids that are present in higher concentrations in light roasts than dark roasts. If you’re a dark roast lover however, darker roasts cause less stomach acid production. A decaffeinated coffee with just 5-25 mg of caffeine will still retain a lot of its healthy chemicals and will provide health benefits in medium and dark roasts.
With these tips you can reduce the caffeine in your body and avoid the rebound of a caffeine withdrawal.
 Coffee Consumption Decreases Risks for Hepatic Fibrosis and Cirrhosis: A Meta-Analysis
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Xie et al “Sleep initiated fluid flux drives metabolite clearance from the adult brain.” Science, October 18, 2013. DOI: 10.1126/science.1241224