How does our body wake up on it’s own from sleep without any external stimuli?
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There are two systems that govern sleep.
The first is sleep-wake homeostasis or also called a sleep drive. It works simply by getting more sleepy the longer you have been awake and getting less sleepy the more you have been asleep. It uses a hormone that builds up and dissipates. Caffeine works but blocks the receptors from this hormone. When babies are first born, this is the only system that is “on”.
The second system is circadian rhythm this uses cortisol and melatonin. This keeps you awake when the first system would have you sleep or asleep when the first system would have you awake.
In a typical person, they start producing melatonin when sun goes down, but it takes until 10-12, where most people probably go to be. It generally peaks between 2-4. Around 2-4 is generally when sleep-drive has worn off. At 4, the body stops producing melatonin cortisol, which starts to get made, usually peaking about 30-40 minutes after waking. So in the morning, you wake up because the sleep drive is low, melatonin is low, and cortisol is high.
Also there are sleep cycles. You go through 3 non rem and one rem stage if sleep. 1 and 2 are light stages and you do some environmental checks before going into stage 3, which is deepest sleep and is where sleep drive dissipates the most. REM happens next. Followed by stage 1. So when you get to stage 1 and you do environment checks you may notice that it’s no long night. At the beginning of the night the cycles are shorter and stage 3 is the longest. As the night goes on, the cycles are longer and rem is the longest. So, a lot of times between 4ish and 6/7 is the last longest cycle too.
If you are taking a nap in the middle of the day you usually wake up just because the sleep drive is depleted a bit. But also because you have done a cycle of sleep.
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