Nothing quite like that morning cup of coffee to get your brain cells turned on and make you feel like a normal person again, right? If you drink coffee every single day then that is pretty much exactly what is happening, you are going to feel normal rather than stimulated because the brain adapts to consistent caffeine intake.
Caffeine binds to adenosine receptors on brain cells. Adenosine is a neurotransmitter that normally causes fatigue, it builds up during the day’s metabolic activities and eventually you need to sleep. Caffeine blocks this process and prevents the adenosine from making you tired and this gives you a stimulant effect. Awesome.
However, the brain is a super cool thing that figures this stuff out, it always wants to be in “homeostasis” so when caffeine consistently blocks adenosine receptors, the brain just increases the number of these receptors so caffeine can’t block them all. Then without the coffee you can feel more fatigued than normal because of all the extra adenosine receptors.
Therefore, to get the benefit from caffeine it is best to only have it sometimes, when you really want the extra boost, but not so often that your brain adapts.