See, you don't actually fall asleep all at once. It's more of a well-defined process that everyone makes their way through in their own time. I'll attempt to explain it.
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You start off moving into what's called Stage 1, from being completely awake.
When you first lay down to go to sleep, you pass from your awake state to what's called Alpha state. You've daydreamed before, right? That's basically Alpha state. You're still mostly conscious, but you start to see some little bouts of color behind your eyes (hypnogogic hallucinations) and you start to feel more relaxed.
After Alpha, you enter Theta state. Theta state is when you could technically be considered asleep. This is when you move completely into sleep paralysis (have you felt like you were falling then woken up with a start? that's sleep paralysis setting in and you not being completely unaware when it happened). You're still sleeping relatively lightly, but if you can get through this stage you move into deeper sleep.
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The next major stage of sleep is called Stage 2.
Your brain starts to produce short periods of rapid brain waves that are called Sleep Spindles. This is the precursor to what comes next in sleep, deep sleep. Your body temperature begins to drop and your heart rate slows down, settling you in for the night.
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You then enter Stage 3.
When you enter stage 3, really slow brain waves called Delta waves start happening. This is the true transition between light and deep sleep.
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You then enter Stage 4.
Stage four is commonly referred to as Delta Sleep because of the brain waves associated with it. It's a deep sleep that lasts for about thirty minutes. You are the technically the most asleep in this stage.
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Here's where it gets interesting. There is another stage of sleep unlike all the others. It's called Stage 5, or REM sleep.
REM stands for Rapid Eye Movement, your eyes literally start moving rapidly behind your eyelids. Your breathing rate increases, and your brain waves shoot up to almost as active as if you were awake. Your voluntary muscles become completely paralyzed, and you start dreaming. REM is also where most of the body's repairs happen. REM sleep is what makes you feel rested when you wake up, and where the body actually rests and restores itself (which is why you usually don't feel rested after a night of drinking or smoking weed, they both inhibit REM sleep).
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These stages come in cycles. It takes roughly 90 minutes for you to go from awake into REM. The first cycle typically has a short REM period, but subsequent cycles increase the duration of REM.
When we actually fall asleep, we go from stage 1 into 2, then 3, then 4, but here's where something curious happens. We then go from 4 to 3, then 3 to 2, then into REM. After REM, we usually return to stage 2, then go back to 3, 4, then 3, 2, REM. We have as many cycles as we stay asleep for, when we wake up, feeling refreshed and ready or the day.
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I don't know if that answered your question, but I hope it shed some light on the subject.
Edit: formatting.
Edit 2: I'll be here if anyone has any other questions they'd like answered about this topic.
Edit 3: Holy crap! Thanks for popping my gold cherry anon! Much love! <3
Edit 4: Wow this blew up... Thanks for all the questions! I'm getting to them all, don't worry!