How does falling asleep actually work; why can you be mid-thought and just conk out?
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Sleep is controlled by 2 systems. First is sleep-wake drive. Its pretty simple the longer you have been awake, the sleeper you get, and the longer you have been asleep the less sleepy you get. A neurotransmitters called adenosine builds up during the day and dissipates at night. A new baby only has this system, so it’s one of the reasons a baby wakes up and sleeps frequently.
The second is circadian rhythm. It will keep you awake when the first system would have you sleep and it keeps you asleep when the first system would keep you awake. It uses melatonin and cortisol for this. Circadian rhythm usually develops around 3-4 months old.
You fall asleep in what is called a sleep gate. The sleep-drive wants you to sleep and circadian rhythm is producing melatonin.
Now when you first fall asleep you enter stage 1 sleep, and even stage 2 sleep, you might not notice that you have transitioned to sleep yet. When researches would wake patients up during these stages, they would say they hadn’t fallen asleep yet. This is even more true if you have insomnia. So you may still continue your line of thinking at this time.
It’s only when you get to deep stages of sleep that you would “conk-out.” During this delta wave/third stage is when the adenosine is cleared and the brain is the most asleep. So, your frontal cortex is off, and so is memory making. So even if you did continue your line of thought, you wouldn’t remember it. The next stage is REM. Some of the brain is more active at this stage, but the frontal cortex and memory are still off. After REM, you would briefly go back to stage 1 and 2. Do a surroundings check which you may or may not remember the next day. Repeat
At the beginning of the night, the cycles are the shortest, and the delta wave sleep is the longest portion of the sleep. As the night continues, the cycles get longer, and REM is the longest portion.
During the day for a nap, it works differently and it will depend on when you fall asleep. The sleep foundation can explain it better.
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