system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What's happening in my body when I take energy shots like 5-hour energy?

I've been taking them for so long and have always wondered the long term effects. It always feels like a weird form of undeath. Where I can't close my eyes but I know I'm still tired. Am I borrowing time from future me to power current me?
gpt
Caffeine is classified by drug scientists as a "Adenosine Receptor Antagonist". In your brain, the neurons release waste products as they work, and one of these waste products is Adenosine. Your brain has Adenosine Receptors that gauge the level of Adenosine in your brain fluids, and when the level of these waste products gets too high, you brain gets sleepy--as most of the waste cleaning action occurs when you sleep.

Caffeine is a molecule that resembles Adenosine and thus can fit on to the Adenosine receptors and turns it off (receptor antagonism). Without the receptors active, your brain is unable to get to sleep--so your body may be tired, but your brain is unable go to sleep.

As for the rest of the stuff in energy drinks (e.g. taurine, gensing) testing do not show that they have any effect on alertness.

After drinking so much caffeine, you have reached a state of tolerance toward the caffeine, where your brain has essentially adapted to the constant level of caffeine in your system. So for you, normal = caffeine, abnormal = no caffeine. Caffeine also affects other systems in the brain, such as the GABAnergic, serotonergic, and dopaminergic systems, and upsetting those systems leads to the symptoms of caffeine withdrawal (i.e. the twitching, anxiety, etc).