Mirroring what others have said, often even heavy sleepers have sounds there sensitive to that wake them up immediately. I had someone crash on my couch one night. I knew they had to be up in the morning for work, so I set an alarm for myself to make sure they got up too (this is right around when smartphones were starting to become a thing, and it wasn’t really common to have alarms set on your phone, hence why I was going through the extra effort for them). I woke up and went over to try and wake them up. First it was a soft nudge in the shoulder, then a harder one, then a shake, then grabbing them by both shoulders and aggressively shaking them. I literally stopped to check their breathing and make sure they hadn’t died on me. My roommate came out during this and was also laughing and wondering if they had died. I really had to use the restroom, so said I was going to do that and then come back and give it another go. While I’m in there, I hear their phone go off and my roommate starts laughing hysterically. Apparently as soon as the tone went off on their phone, they sat upright, wide awake.

I dunno that it’s cut and dried.

Me? I’ll sleep through storms and music and loud cars. But if you even tiptoe into my room, I’m awake and usually already moving by the time you’ve taken a few steps.

However, different people cause different levels of response. My wife and kid can usually get close to the bed, maybe even reach out to touch my leg. My best friend is the same way. My dad, and my best friend’s husband I wake as soon as whatever it is that my brain recognizes is in the room, but I’m not on edge the way I am with anyone else.

My dad is similar about sounds outside the house, he’ll snore through a hurricane, and has. But if you speak his name quietly outside his door, he’s awake and talking to you, ready to move if he needs to.

My wife though? Fuck me, she sleeps hard. I have had to shake her awake more than once.

The kid is more like my dad with their name being called, but they also wake up to any loud noises outside. They will, however, sleep through an entire conversation right outside their door.

I believe the reason is that our brains filter senses all the time, awake or asleep. Everyone filters a little differently. So it makes sense that we’ll all pick up habits of filtering things our brains have learned are low priority, but rouse us for higher priority things. What’s crazy to me is that those filters don’t always make sense.

When I was a teenager I slept through a fire alarm once. I didn’t wake up until my mom walked in the room. I woke up before she said anything. Just her existing in my room for 0.5 seconds woke me.

So… idk.

My all accounts, I’m a heavy sleeper, there are basically only 3 things that will reliably wake me up

My alarm clock
Having to pee
My dog throwing up or whining to go out (usually an indication that she’s gonna have diarrhea)

I sleep through my wife’s alarm going off (usually several alarms, she like to hit snooze,) showering, turning lights on, listening to podcasts while she gets ready, the sun coming up (I work partially overnight, I’m usually in bed by about 4 or 5 AM,) landscapers mowing the lawn outside my window, kids screaming at the nearby playground and school, fireworks, thunderstorms, construction (although I was not able to sleep through the siding repair I had done with a guy hammering on the wall directly behind my bed)

One time my wife was able to get me out of bed and stand me up so she could fix the sheets without me being fully awake.

When I was a kid my mom could vacuum in my room without waking me up.

I don’t know what this makes me, but I sleep through my phone going off and being touched (poked).

A light sleeper just means that your sleep is easily disturbed by small changes in your surroundings (like the turn of a doorknob, a car passing by outside, a text message to a phone on vibrate). There isn’t a strict criteria for it.