The world of the mouth, and of saliva during sleep, is a rather disgusting deal no matter how you cut it. The purpose of circulating saliva, in sleep as in waking, is to redistribute and cleanse bacteria that gathers in the mouth cavity. The saliva produced by the mouth's salivary glands contains antibacterial compounds that aid in cleaning away build-up in the mouth, though proper oral hygiene does mean that you need to supplement it with mouthwash, flossing, and toothbrushing. (Sorry.)

This is where it gets a bit disgusting, though. Saliva levels in the mouth in sleep are tied to our swallowing reflex, which helps to regulate the amount of saliva present. In some of us, the swallowing reflex relaxes deeply during sleep, meaning that saliva builds up and can spill out into drool on the pillow. In others, specifically those of us who sleep with our mouths open thanks to position or nose blockage, swallowing may continue as normal, but saliva dries up, meaning that bacteria no longer get their usual cleansing bath and accumulate rapidly across the surfaces of the mouth, including the tongue. This is the source of the famed morning breath, a smell related to built-up bacteria developing overnight, and is particularly bad for people who experience dry mouth because of medical conditions or medications, like certain antidepressants.

As a standard part of deep REM (rapid eye movement) sleep, humans experience what's known as complete muscle paralysis: the inability to move any muscle in your body except for those related to eye movement. This appears to have evolved for human safety, as dreams may inspire movement (kicking, running, fighting) that could endanger us or those around us if our bodies allowed us to act them out. Scientists only isolated the mechanism behind this paralysis in 2012, and it's pretty creepy: two immensely powerful brain chemicals interact to make it completely impossible for us to move during this state.

Those people who experience the condition sleep paralysis are experiencing the effects of this chemical interaction while they happen to be waking or falling asleep. Sufferers lucid enough to report what it feels like report utter terror at their realization that their body refuses to allow any movement except for their eyes. It's not known precisely why sleep paralysis develops, as humans are clearly not meant to be awake while this experience occurs, but it's probably the most terrifying sleep condition around.

The "sleep" that accumulates and dries at the corners of your eyes as you sleep is not, in fact, fairy dust or whatever your parents explained it to be. It's a collection of mucus, oil, dust, and other ingredients that help the eye's cleanliness and hygiene. Normal eyes have three layers over the surface of the eye: an underlying protective mucus membrane, a layer of water-based lubrication, and then an outer layer made of a substance called meibum, which is composed of oils and sebum. The meibum and the water layer usually mingle gently to provide the eye with maximum moisture and help banish things like dust and irritants. As it cools overnight, meibum becomes white and thick, forming the crust at the corner of the eye that we find so familiar.

Looking at the eye gunk itself, however, can be a good health alarm for issues with your eyes. Too much gunk can mean you have an eye infection, while variants in the discharge can highlight issues like allergies or sinus problems. Yes, it is perfectly legit to go to a doctor because your eye crusts look funny to you.

We're all familiar with the regenerative elements of sleep, but it's particularly important for skin, which continues its normal everyday process of cell regeneration at night by sloughing off dead cells and replacing them with new ones. This is part of the reason you shouldn't sleep with make-up on; it traps the dead cells on your rather than shedding them. It's also why it's necessary for you to wash your pillowcases and pillows regularly; it's estimated that up to a third of your pillow weight can be composed of dead skin cells and dust mites.

The body's immune system tends to take advantage of the overnight rest to heighten production of certain proteins that ward off infection, including one with a seriously creepy name: tumor necrosis factor. TNF is what it says on the tin: it kills tumor cells, and is deeply involved with the body's regulation of cell proliferation and cell death. When it goes wrong, scientists allege, everything from Alzheimer's to cancer could result. It's also got a deep relationship with our sleep patterns. Sleep deprivation, in a 2011 experiment, induced a spike in TNF levels, and studies have shown that it seems to help create sleepiness in the brain. Sleep, it seems, is the time when the body's killer army comes out.

There's a spectrum of odd activity that can occur with your limbs at night. At one end is periodic limb movement, which is extremely common, particularly as people are falling into deep sleep, and involves the involuntary flexing or retraction of a muscle, producing shivers or "kicks". At the other, more extreme end is the disorder known as restless leg syndrome, which affects up to 10 percent of adults and means that the body is so extreme in its night movements that it continually disrupts sleep and can make life a misery. Scientists aren't entirely sure what the body is doing with these movements and how they're caused, though they seem to be related to the nervous system, but research as yet remains inconclusive.

Did you know that your stomach secretes around two liters of acid every day to help you digest food and break it down into nutrients and waste? The human body is so weird. That acid production, though, appears to be in tune with our circadian rhythms and REM sleep. It peaks radically between 10pm and 2am, pumping out huge quantities of hydrochloric acid to help the digestive system churn into action and dissolve food matter. The cause of heartburn during sleep is usually the relaxation of a ring of muscle called the transient lower esophageal sphincter (try saying that five times fast), which lets the acid escape up into the esophagus and cause a really unpleasant burning sensation.

The one "disgusting sleep" fact you've likely heard in your life is the whole "people swallow seven spiders a year (or in their entire lives)" one. But are spiders that colossally stupid, and are humans that likely to get them in their mouths? The answer is no, and the reason behind the proliferation of the myth itself is a hilarious and complicated one. It's alleged that the first hint of this statistic came from an article in PC Professional magazine in 1993 in which the journalist made it up to show how factoids delivered by email exposed people's gullibility (a kind of precursor to the Nigerian Prince scams). However, nobody has yet been able to find this article or trace the origins of it, so this element itself may be another urban legend added onto the idea.

Humans are unlikely to eat spiders in their sleep, spider experts maintain, because spiders themselves have very little interest in the human face unless they encounter it by accident, and are quite capable of using heat and motion detection to avoid the giant snoring thing. If you do happen to chow down on a spider as you dream, you're not one of a majority.