system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening inside your brain when you are drowsy?

Example: When you are struggling to stay awake in the car and your head starts nodding off but you try really hard to stay awake, what is happening? How is the brain able to stay awake when if feels the urge to sleep so strongly?
gpt
A simplified answer: while counteracting sleep pressure (or drowsiness or sleep deprivation), parts of your brain that govern attention become more and more active as you try to sustain attention. This 'compensatory' activity in these regions also drives activity in parts of your brainstem that promote overall arousal. 

These brainstem arousal centers in turn generate wakefulness in the rest of the brain, like the cortex. They also serve to inhibit inhibit sleep-promoting centers of the brain. This enables sustained attention despite sleep pressure. But eventually, with enough sleep pressure, no amount of compensatory activity will prevent lapses in attention.