If you've ever shared a bedroom with someone, you're probably aware of the fact that some people can sleep so deeply that you could blast the TV at full volume and they wouldn't even stir, whereas others require the perfect cocktail of darkness, silence, and temperature to keep them asleep. You may even be both of those people in a single lifetime—or even in a single week. But what exactly makes someone a light sleeper or a heavy sleeper? Turns out, the answer is fairly complicated: Sleep, like most other facets of human life, involves a lot of factors.

"Assuming everyone has a healthy lifestyle, there is individual variation," says Jocelyn Y. Cheng, MD, assistant professor of neurology with a specialization in sleep medicine at NYU Langone. So even after ruling out differences in lifestyle—diet, activity, substance use—and disorders like sleep apnea, our sleep habits can still vary greatly.

But what is "deep sleep" anyway?

We all go through cycles of deeper and lighter sleep every night, so the difference between a heavy and a light sleeper may be the amount of time a person spends in certain phases of their sleep cycle.

As you may remember from high-school biology, there are four stages of sleep that we cycle through every night, from the lightest (falling asleep) to the deepest, wherein the body repairs and strengthens the muscles, brain, and immune system. After each cycle, we enter REM (rapid eye movement) sleep, which is where dreams happen. The NREM (non-rapid eye movement) stages leading up to REM take about 90 minutes, and each REM period lasts a bit longer than the previous one, usually up to an hour by the final REM stage of the night. REM is technically the lightest kind of sleep—closest to waking.

Like many things, sleep is easier when you're a kid.

The deepest stages, however, are "collectively known as Slow-Wave Sleep (SWS)" and are common when you're very young, says Michael Perlis, PhD, director of the Behavioral Sleep Medicine Program at University of Pennsylvania's Perelman School of Medicine. "The large amounts of SWS in early life likely accounts for the deep sleep that young people experience."

This may explain, in part, why, when you were little, your parents may have been able to carry you from the couch to your bed without you ever knowing, but as an adult, the slightest jostling from your partner might pull you right out of sleep.

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Other factors—including lifestyle—play a part.

"Sex, metabolism, and genetics also likely determine how deeply a person sleeps," Perlis says. Studies have found that women tend to preserve their slow-wave sleep better than men. However, there is an overall decline in slow wave sleep as we age, but there is more decline in men overall.

Hormones and other brain functions are also part of the sleep/waking cycle, including systems that specifically keep you from waking up. And when you're awake, sleep-promoting substances accumulate in your brain—essentially getting you more and more ready to go to sleep the longer you stay awake.

Your environment factors into your sleep quality too. While Perlis and Cheng both recommend a quiet, dark, cool environment for sleeping, most of us already know how we sleep best—even when that means lights, music, or the TV on.

"If somebody is already habituated to something, they go to sleep when they're tired and wake up when they want to, and it doesn't really interfere with their functioning during the day, it's truly not going to be much of a problem," Cheng says. So if you're used to sleeping with noise or sound in your environment, and it doesn't seem to be negatively impacting your quality of sleep, there's probably no harm in it.

This is probably part-idiosyncratic and part-habituation—no one knows exactly why one noise/light environment works better for some people than others, except that you generally sleep best with whatever you're used to. Perlis adds, however, that "predictable or monotonous noise" can help mask unexpected sounds in the night to help prevent sudden waking for those of us more sensitive to noise. White or pink noise, he says, is generally better for this function than television.

You may just be more (or less) sensitive.

"Part of this may have to do with the individual's neurobiology—their strength of sensory inhibition during sleep," Perlis says.

"For reasons we don't understand, some people just happen to be more sensitive to certain provoking factors. Some people tend to be very sensitive to light, and that's not uncommon, because light is one of the strongest zeitgebers (environmental clues) for keeping us awake," Cheng says.

Other atmospheric factors also play a part in shaping a person's circadian rhythm. But ultimately, people are different: Just like genetics and other inbred factors can hard-wire what makes some of us happy running on six hours of sleep while others can't function without a full eight, some of us are just naturally more sensitive sleepers. If you're one of these, that doesn't mean all hope is lost, however: There are definitely a variety of ways to help yourself sleep better—whether or not you lucked out in the genetic sleep lottery.

Ariana DiValentino is a writer and filmmaker based in Brooklyn. She is very, very worried. You can follow her on Instagram and Twitter.

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