system
Given the following post, respond as if you're commenting on Reddit:
human
When you wake up by yourself after a period of time (be that 7 hours or 12), is that the amount of sleep your body required?
gpt
The simple answer to your question is: no, not necessarily. A lot of it depends on the timing.

In terms of the biological factors involved, sleep timing can be largely understood in terms of two processes:

1) Your circadian rhythm, which times your 24 h rhythm of sleepiness, hunger, hormone release, etc.

2) Your sleep pressure, which increases the longer you are awake and decreases the longer you are asleep.

In addition, environmental influences, such as light and noise, may alter your sleep patterns and your likelihood of awakening, but let's forget about those for the moment. Sleep timing can then be understood quite well in terms of the above two processes.

Your circadian rhythm promotes wakefulness during the biological day and sleep during the biological night. That means that even if you have a very high sleep need, you may not be able to sleep well during the day. This is something that often affects shift-workers. 

For similar reasons, it may not be possible for some people to sleep in, even with a large sleep debt, due to the wake signal provided by the circadian rhythm around the normal waking time. This also means that if you manage to fall asleep earlier than you usually do, you can sometimes sleep all the way to your usual wake time.

Now, to get to your specific example, sleeping for 12 hours continuously suggests either or both of the following: (i) sleep was initiated much earlier than usual; (ii) there was a pre-existing sleep debt.

It is also worth noting that one night of increased sleep duration is insufficient to compensate for a sleep debt built over weeks.