system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 what does caffeine actually do to make you feel awake?
gpt
Hi. Doctor here. Caffeine acts through multiple ways to enhance alertness. 

1. It activates sympathetic nervous system, giving you increased blood pressure, elevated heart rate and an energy boost. 

2. It dulls the brain's ability to sense when the body is tired. 

Context: The body breaks down Adenosine Triphosphate (ATP) to release energy. With time the Adenosine portion of ATP builds up as more ATP is broken down throughout the day. This buildup of Adenosine tells the brain that the body is tired and needs sleep to replenish the ATP reserves, The brain then releases Melatonin, which makes us sleepy.  

This adenosine-to-brain communication is disrupted by caffeine, tricking the brain into thinking the body is not tired yet, so it keeps going. 

Hope this helps.