Brewed awakening: What time of day you regularly drink coffee affects how you feel.
Brewed awakening: The timing of your regular cuppa affects the way you feel.Photo: Joe Armao
There are two types of people in the world – early-morning coffee drinkers, who fire up some kind of brewing apparatus as soon as they get out of bed, and mid-morning coffee drinkers, who get their first hit a bit later. And depending on which type you are, the caffeine may have a greater or lesser effect on how you feel.

We all know caffeine is addictive. The latest edition of the Diagnostic and Statistical Manual of Mental Disorders lists caffeine withdrawal as a mental illness. But how does caffeine work? Here's some caffeine pharmacology, courtesy of The Smithsonian Magazine.

Because it's water and fat soluble, caffeine is able to enter the brain. It's similar in structure to adenosine, a byproduct of cellular respiration that blocks the brain's adenosine receptors and produces the feeling of tiredness.

Caffeine also blocks these receptors, locking the adenosine out, and stopping the tired feeling. What's more, the brain's natural stimulants – such as dopamine – work better when the receptors are blocked, and the surplus adenosine also triggers the adrenal gland to produce adrenaline – another well-known stimulant. So caffeine is also an enabler of other stimulants.

Caffeine interacts with the body's circadian rhythm, which regulates when we sleep and eat, in interesting ways.

Stephen Miller, a PhD candidate at Uniformed Services University of the Health Sciences in Bethesda, Maryland, notes in a blog post that as part of the circadian rhythm, the concentration of cortisol (the stress hormone) in our blood rises and falls.

One of the periods of peak cortisol concentration, on average, is between 8am and 9am each day. The concentration then drops between 9.30am and 11.30am, before rising again.

Miller argues that ingesting caffeine when cortisol levels are high produces a reduced effect – so those who reach for a cup of coffee mid-morning will benefit from a greater increase in alertness than those who drink coffee first thing, when their cortisol levels are high.

Miller notes that cortisol concentration drops off again after 1.30pm – that'll be the afternoon slump that sends many of us scurrying for another quick shot.

When you have caffeine every day your brain grows more adenosine receptors, which eventually means you need a bigger dose to block them all and get the same effect. But if you stop caffeine for between seven and 12 days, the number of adenosine receptors decreases. So the third type of person in the world – the one who goes on regular coffee detoxes – is actually on to something.