Caffeine acts by causing an adrenalin shot into your blood stream. This in turn causes a release of sugar into the blood stream. This "wakes you up". A short while later, this effect wears off. The adrenalin is processed, the sugar is shunted out of the bloodstream, some into the muscles and some into the fat cells.
The body and brain then suffers a crash of energy. This causes dramatic compulsions to have more caffeine or to eat sugary foods. The compulsion is so strong that we succumb. Often, in our world, the nearest option is a caffeine drink or a sweet snack. A while later, the effects wear off, which sparks off our hunger for more caffeine or sugar. A cycle develops that taps into the sugar craving brain tissues and the energy seeking body and mind.
Caffeine is addictive which means that, amongst other things, we can't do without it. Tell a regular coffee or tea drinker to give it up, and they will look at you in horror and say "I can't get through the day without it." An alcoholic says exactly the same thing. Caffeine is responsible for high blood pressure, bodyweight gain (not weight loss as advertised by brands) and for dehydration of the body and the problems linked with that. Despite its energising properties, caffeine reduces the body's natural source of energy, and caffeine addiction has a large number of effects. Google it. Its out there.
Caffeine causes acute single mindedness and prevents clear thought, just as adrenalin does. Most importantly, when it comes to sleep, the human's most powerful and important regenerative process for mind and body, caffeine prevents it. The caffeine trap sees people drink caffeine before bed, and first thing in the morning. People are riding a wave of exhaustion, and the body gradually weakens. The addiction is firmly in place. Caffeine is a diuretic, which means it makes you pee. This leads, actually, to dehydration and leaching of minerals and vitamins. Vitamins not replaced in the next caffeine hit.
When a person who avoids caffeine drinks some, he or she will find sleeping properly hugely difficult for the next 6-8 hours. The next morning, they feel tired. This can escalate into serious symptoms of fatigue - sometimes chronic - if unchecked.