Do you catch heavy Zzz’s or take a heavy L when it comes to sleep? Do you snooze or lose?

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 drive a combine harvester into the room and they wouldn’t even stir.

Others require the perfect cocktail of darkness, silence, and temperature to keep them asleep. Disturb the balance and POW! It’s wakey-wakey time and a full night of tossing, turning, and shuffling.

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You may even switch between both modes 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 (ugh, thanks science). 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, even how late we like to watch TV — and disorders like sleep apnea, our sleep habits can still vary greatly.

We get under the sheets with sleep habits to work out what makes us stop ticking for the night.

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. No-one is 100 percent a deep sleeper or 100 percent a light sleeper.

There are four stages of sleep that we cycle through 4 to 6 times every night, from the lightest (falling asleep) to the deepest, wherein the body repairs and strengthens the muscles,Dattilo M, et al. (2011). Sleep and muscle recovery: endocrinological and molecular basis for a new and promising hypothesis. brain,Eugene AR, et al. (2015). The neuroprotective aspects of sleep. and immune system.Besedovsky L, et al. (2012). Sleep and immune function.

Within each sleep cycle (which is approximately 90 minutes long), there are 3 stages of non-rapid eye movement (NREM) sleep and then rapid eye movement (REM) sleep.

REM sleep is where dreams happen. We know you’ve probably been told that Disneyland is, in fact, where dreams happen, but we’ve got news for you — REM is cheaper. And you hit it up every night, multiple times.

Each REM period lasts a little longer than the previous one, usually reaching up to 1 hour by the final REM stage of the night.

REM is lighter than deep NREM sleep. So if a person spends a whole lot of time stuck here, they’re going to be easily roused from their slumber.

NREM sleep is divided into N1, N2, and N3 sleep. N1 is very light, occurring as you just fall asleep. Most of the night is spent in N2 sleep. The deepest sleep is N3 sleep and occurs more during the earlier sleep cycles and less during later sleep cycles.

If you’re one of those people that has a less easy time with staying in deep sleep, we rounded up seven gadgets that might help.

Deep or N3 sleep is “known as Slow-Wave Sleep (SWS),” and is easy to stay in 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.

However, as an adult, the slightest jostle or night-time fart from your partner might pull you right out of that dream you were enjoying with the jet ski.

We ain’t in SWS-town anymore, Dorothy.

Most people need a full dose of sleepy time — here’s why.

Let’s be real, here: You’re going to have a harder time drifting off at the end of a cocaine bender than after half an hour of meditation and a good book. But it’s not only lifestyle choices that affect your ability to tally up sheep.

“Sex, metabolism, and genetics also likely determine how deeply a person sleeps,” Perlis says.

Studies have found that biological females tend to preserve their slow-wave sleep better than males.Mallampalli MP, et al. (2014). Exploring sex and gender differences in sleep health: A society for women’s health research report. However, there’s an overall decline in slow wave sleep as people get older, but more so in males.

Hormones and other brain functions are also part of the sleep/waking cycle, including systems that specifically keep you from waking up.Mong JA, et al. (2016). Sex differences in sleep: Impact of biological sex and sex steroids.

And when you’re awake, sleep-promoting substances build up in your brain — essentially getting you more and more ready to go to sleep the longer you stay awake.Brown RE, et al. (2013). Control of sleep and wakefulness. Long days feel exhausting for a reason. Your body knows you need a top-up.

Your environment factors into your sleep quality too. (If you’re in a blisteringly hot room, it can affect your sleep swag — we looked at loads of ways to cool down a room that help you snooze.)

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 having 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. It’s why “Friends” is so easy to have on in the background and sleep to.

This is probably partly down to individual differences between people and part down to environment and learned behaviors. 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. We filtered out the static to find you the best sounds for sleeping.

“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 sleep cycle. But ultimately, the answer is pretty simple: People are different.

We had a look at whether you should sleep on an empty stomach.

Just like genetics and other factors can hard-wire what makes some of us happy running on 6 hours of sleep while others can’t function without a full 8, some of us are just easier to startle awake.

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.