system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when you wake up well rested, you’re conscious of time having passed, but when you don’t get enough sleep, it feels like you had just fallen asleep?
gpt
When you sleep for a full 8 hours (or whatever a full cycle is for you, everyone is different) your body goes through three distinct stages of sleep. Stage one is mostly just dozing off, while stage 2 and 3 sleep is where your body gets its deepest rest, and repairs itself most effectively. Stage 3 specifically is known as REM sleep, or Rapid Eye Movement. This is the stage where you dream, and where you are physically and mentally disconnected from the rest of the world.

As you sleep for the whole night, you fall in and out of stage 2 and 3 more quickly, until your body starts producing adenosine to kickstart your brain and wake you up. You slowly go back into stage 1 sleep, and then you’re awake.

When you get awoken in the middle of your deep sleep, your body has to go from a deeply committed biological cycle that takes hours to complete, to alert and functioning, immediately. Depending on which stage you were in, your body may have induced sleep paralysis to prevent your dreams from making you wander around in your sleep, which it now has to undo *right now*.

It’s like trying to drive a car in very cold weather without warming it up. The cold car (sleeping person) has all of its oil still stuff and viscous, and driving the car like that is going to be harder, whereas the car that was preheated and properly allowed to warm up is much easier to operate.