system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the biology behind being a light sleeper vs a heavy sleeper?
gpt
Very little. There are all sorts of studies where people in sleep studies claim terrible or awesome sleep, yet their sleep data is very similar.

Much of how we understand our own sleep comes down to bias and perception. If you sleep through something, you don't remember it. You only remember the times you woke up, yet some people use that to "prove" that every little noise sets them off.

You normally cycle through four stages of sleep every night. At NREM 1, you are right in the fuzzy grey zone of awake and asleep. Your brain is still aware of a lot of external surroundings (sounds, sleeping next to someone who tosses and turns all night) and you are easy to wake up in that stage. When you hit NREM 3, you are pretty much dead to the world, scientifically speaking. It takes a lot to wake you up, your brain really wants to ignore everything, and if you do wake up in this stage, you will feel gross and kind of out of it. Since you hit these stages more than once a night, you will naturally be more awake or more asleep at various points in the night. That's normal and healthy. People naturally spend less time in NREM 3 and REM sleep as they get older.

People who experience hypervigilance (as in subclinical anxiety problems, anxiety disorders, or disorders related to trauma like PTSD) are "scanners." This (a) has a *huge* impact on quality of sleep, and (b) makes them very sensitive to any sort of sleep disturbance, which makes the problem seem a lot bigger than it really is because it means interpreting normal sleep functions (hitting light sleep stages during the night) as a problem or threat.

That there are tons of sleep disorders, mental illnesses, physical illnesses, etc. out there that mess with normal sleep architecture, so for those people, they may spend more time in light sleep stages or otherwise find their sleep getting a bit messy and therefore be more likely to wake up. Some sleep medications (especially benzos and z-drugs like zopiclone) also mess up sleep architecture, ironically. Regular old alcohol and other drugs can have similar effects.

ETA: If you consistently have poor sleep, talk to your doctor!