Coffee is an essential part of many people’s wake–up routines. Partway through the dark, bitter beverage, that drowsy–morning feeling is magically transformed into perky energy, and the day can officially begin. But coffee works a little bit differently for me and many others. When it becomes time for me to do homework, I brew a cup to calm my buzzing mind and focus on the task at hand.

So, how does caffeine wake us up in the morning? And why does coffee energize some but focus others?

You likely already know the active ingredient in coffee, tea and other energy-inducing beverages: caffeine. This commonly consumed drug acts as a stimulant within the body, which reduces drowsiness and replaces it with alertness. It accomplishes this by altering some chemical levels in the brain: increasing dopamine and blocking adenosine.

What’s dopamine? It’s in a class of molecules that is responsible for sending messages throughout the brain and body. Dopamine specifically is often associated with pleasure and rewards, but it’s also important for reinforcement, motivation and focus.

And adenosine? This is a chemical that slows down your brain activity and helps with sleep. Upon waking in the morning, your body still has a lot of active adenosine that’s trying to convince you to retreat to your cozy bed.

To put all that together, the caffeine in your coffee stops the flow of sleepy adenosine and increases the action of the motivating dopamine, thus giving you an energy boost to greet the day.

Second cup down and you may notice restlessness and irritability setting in. The trick with dopamine is the narrow range in which prime focus time occurs, and you’ve likely overshot this window once you’re racing and jittery. Be wary if you’re still drinking come afternoon because refueling on energy now will interfere with a good night’s sleep later.

Let me wrap up by addressing my second question because coffee doesn’t affect everyone the same way. For people with ADHD and related conditions, caffeine focuses an otherwise fast-paced distractible mind. Caffeine’s stimulant effects, in this case, come back to dopamine. Folks with ADHD typically have lower than normal dopamine levels, which means that a few cups of coffee will bring dopamine up to a normal range optimal for focus.

In the end, remember this: Everyone’s body chemistry is different. Just because coffee helps me focus and helps my friend wake up doesn’t mean that it will do the same for you.