Coffee

Caffeine is the world's most widely consumed psychoactive drug - mainly owing to how it enhances mood and it's stimulatory effect - and is predominantly imbibed via coffee.

Let's dive into a little bit how it actually works for us.

It all starts with adenosine.  Adenosine is a neurotransmitter (basically a chemical messenger for our brain) which encourages sleepiness as the day goes on and as it's levels build up and it attaches itself to certain receptors in our brain.  There are multiple receptors in the brain, adenosine locks onto one called the A1 receptor.  When adenosine locks with the A1 receptor it promotes muscle relaxation and sleepiness - hence why we get more tired as the day progresses.

Adenosine is naturally produced in the body (in fact in pretty much every cell) driven by physical activity and intensive brain use. Over the course of the day the level of adenosine increases.  Once asleep, your body naturally metabolises away the adenosine, enabling, once the grogginess after sleep disappears, the body to start a new day of activity as sleep has been restorative.

Let's assume you are like me, and soon after waking you consume your first cup of coffee.  Caffeine is absorbed in the small intestine within an hour of consumption and quickly makes it's way around the body in your bloodstream.  As it starts entering your brain then it starts to compete with adenosine and blocks adenosine locking with the A1 receptor.  This can result in a jolt of energy.  It also gets in the way of another receptor called the A2A receptor which helps promote the release of dopamine and glutamate  - this makes you feel good after drinking your coffee!

Eventually the caffeine molecules dissipate (they are actually metabolised into other chemicals by enzymes) allowing adenosine to bind with the A1 receptor and barring other events, lowering the levels of dopamine and glutamate as it no longer blocks the A2A receptors. 

It can take up to 10 hours for the caffeine in a single cup of coffee to be metabolised.  So, by early evening most of the caffeine is gone and adenosine (which has continued to build up in your body) can go about it's business and bind to the A1 receptors.  Now you begin to feel sleepy.

Everybody metabolises caffeine at different rates - indeed some people can consume caffeine quite late without a significant impact on their sleep.  For most of us, it is not recommended to consume caffeine too late in the day as the blockage of the A1 receptors will continue into the night which impacts the quality and quantity of our sleep.

Sleep is a vital part of this process.  Adenosine encourages the brain to sleep which allows the body's restorative cellular processes to kick in, preparing is for a new day of physical and mental activity.  Poor sleep quality, from consumption of caffeine (or other reasons such as alcohol consumption, stress etc) can turn into a vicious cycle as we consume more caffeine the next day to help keep us alert whilst the body and brain needs to sleep.  Long term poor sleep quality can have serious negative impacts on our bodies and has been shown to contribute to a wide number of diseases - physical and cognitive.  So, drink your coffee wisely and enjoy it!