system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Does sleep "stack?"

Say i went to bed from 5pm-9pm, woke up, and went to bed again from 2am-6am. Would that be the same as sleeping for 8 hours at night? If not, how are they different?
gpt
In short no.

So sleep is not one big chunk. It is rather like a series or cycle of smaller things.

Sleep progresses through stages so you normally go into initial sleep which is just a restorative state where you can wake pretty easily. Then comes light sleep where your body actively starts resting followed by deep sleep and then REM. After REM you cycle back to light then deep etc until you wake up. On average each cycle takes about 90 minutes

So when you sleep, you are stacking cycles not a whole block of sleep. 

Some people sleep one block per night(normal 8 hours although sleep amounts vary from about 6 which is seen as a sort of min to 10 or 12hrs), others sleep a four hour block then awake for a while then sleep again then awake for the day.

Sleep is not bankable BUT being rested means when you need to push hard, it is easier to prolong wakefullness whereas when you are chronically tired , you struggle to keep pushing.

It is also very important to stick to a sleep schedule, even on weekends. It is also possible to support lack of sleep with naps. Nasa conducted a study on pilots and found that even with a full nights rest, having a micro nap of 5 to 10 min boosted alertness and response times afterwards.

Why does caffeine not work?

Your body secretes a hormone to make you feel tired as a signal that you need rest. Caffiene and this hormone plugs into the same receptor in the brain(imagine 8 usb drives trying to plug into 3 available slots). At a certain point, your body needs rest to clear and restore the hormone imbalance.

This is why coffee naps work.