system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you feel more awake after not sleeping for an extended period of time?

Like if you stay up overnight you start to feel really drowsy at around 24 hours of no sleep, but then after that initial drowsiness you start feeling awake again.
gpt
So there are two independent systems that influences your sleep/wake cycle.

One is the circadian rhythm which controls not only sleep but also your hunger, how much urine you produce, body temperature etc.

The other one is Adenosine, which is a chemical in your brain which creates that sleep pressure you feel when you are sleepy. The amount of Adenosine increase as the number of hours awake increase. This is only recycled from the brain when you sleep.

The key thing here is that this two system do not influence each other, so whether you sleep or stayed up all night your circadian rhythm will still continue as if nothing happens. In a normal circadian rhythm you peak in wakefulness at around noon, so even though your brain is full of Adenosine (which makes you sleepy) you feel awake when your circadian rhythm reaches it's peak.

You feel more tired early in the morning because your circadian rhythm is at low wakefulness and there is a high Adenosine level.