You go to sleep. What happens next?

We all sleep. Every night, we tuck ourselves into bed — or whatever nook we can find — close our eyes, and escape into a faraway land of magic. Here, anything is possible, or so we let ourselves believe.

When you think about it, sleep really is quite strange.

But it wasn’t until 350 BC that Aristotle asked what sleep was and why people did it. And it wasn’t until 1924 ushered in the invention of the brain-wave-graphing electroencephalogram (EEG), that we could even begin to truly understand what happened to us each and every night.

After all, just look at what Thomas Edison, of light-bulb fame, had to say on the matter:

Sleep is an absurdity; a bad habit.

Absurdity, according to the dictionary, is “the quality or state of being ridiculous or wildly unreasonable.” But is sleep really absurd?

Perhaps it is, in the sense that it’s odd and doesn’t intuitively make sense. But it is by no means of the word unique to humans. Here’s what journalist Michael Finkel of National Geographic magazine has to say about it:

Every animal, without exception, exhibits at least a primitive form of sleep. Three-toed sloths snooze about 10 hours a day, a disappointing display of languor, but some fruit bats manage 15 hours, and little brown bats have been reported to laze for 20. Giraffes sleep less than five. Horses typically sleep part of the night standing up and part lying down. Dolphins sleep one hemisphere at a time — half the brain sleeps while the other half is awake, allowing them to swim continuously… Jellyfish sleep, the pulsing action of their bodies noticeably slowing, and one-celled organisms such as plankton and yeast display clear cycles of activity and rest.

Clearly, Edison was wrong. Almost every organism sleeps in some way.

Sleep is so important that even hedgehogs don’t hibernate all through the winter: they interrupt their hibernation in order to sleep.

But that still doesn’t answer the question of why?

To understand why we sleep, perhaps we should see what happens when we don’t.

In the short term, consequences of sleep include: increased fatigue, lack of recall, diminished decision-making and problem-solving capacity. In the long term: increased risk of diabetes, dementia, heart diseases, obesity, learning disabilities in children, depression, and stroke.

The World Health Organisation has even declared shift work, which takes place at odd hours and often on changing cycles, a potential carcinogen.

All this tells us one thing: sleep is important for our bodies in a hundred different ways that we don’t quite understand yet. But the one thing we can understand is, it’s important.

During the day, eighty-six billion neurons are firing rapidly in your head, in order to process the world. Inside your brain, it looks like chaos. Vaguely organised chaos, but chaos all the same.

At night, those same neurons synchronise. They produce long delta waves in choreographed concert. Unlike the short, jagged brainwaves of wakefulness, when asleep your brainwaves have an even, rhythmic, and clear pattern.

What is happening?

The first stage of sleep only lasts about five minutes in the first cycle. This is when you start to drift off, and still get woken up easily. In fact, if you wake up now, you may not even realise you were asleep.

But even this stage doesn’t come on unless the conditions are right. Before going to sleep, your body’s internal lock must be synchronised with the amount of light outside. Your pineal gland, located at the base of your skull, is what triggers the process — and if the light is too bright, it’ll never get into action.

It also matters what kind of bright. White light, the kind found in fluorescent lightbulbs or screens, are worse than more reddish lights. That’s why we have apps like f.lux, which automatically redden your screen in the evenings to avoid disrupting your sleep.

If all this lines up, then your pineal gland gets into action. It begins to secrete the sleep hormone: melatonin. Your body relaxes, and your sensory receptors become muffled.

Sometimes, your relaxing muscles make you feel a sudden jolt, or the sensation of falling — what is known as a “hypnagogic jerk”. People are not sure why it happens: it could be a slight misfiring of muscles, or a leftover from our monkey days when relaxing muscles meant falling off a tree.

But here, you don’t fall off a tree. Instead, if all goes well, you’ll simply fall into Sleep, Stage Two.

Stage Two is when you’re properly asleep. Your breathing and heart-rate become more regular, and your body temperature drops a little bit.

In the brain, you find short bursts of brain activity. On an ECG graph, these bursts look a bit like thread-makers’ spindles — and ‘sleep spindles’ is what they’re called. Each spike lasts half a second long, and stimulates the cortex almost randomly.

Sleep spindles are thought to help save recently-learned information, as well as link it to more important long-term memories.

A study showed that people displayed more spindles on the days they were taught a new task, and that the people that experienced more spindles than others generally did better at that task the next day.

In fact, the researchers have been able to go as far as correlating a higher general intelligence with a tendency to produce more spindles. It makes sense, since spindles literally make connections you might not have thought of making.

Remember those long, rolling brainwaves mentioned earlier? Well, stages three and four, recently condensed into one larger stage, is where they’re most prominent.

This is also when more bodily changes begin to occur. Your core temperature drops a lot more. Your situational awareness is non-existent. You probably wouldn’t even be able to feel pain.

For half an hour, your body is essentially in a coma.

Internally, however, there’s quite a bit that’s happening. Stage three sleep is when your growth hormone is released. Your blood pressure gets some moderation. Hormones that regulate your mood are secreted. Your immunity gets built up.

But there’s only so much of this coma-like state that your body can take. It wakes itself up.

Or does it?

You’re running. You’re flying. You’re falling.

Or at least, your brain is telling you that you are. And even though the most you might have done is turn over, you believe it, too.

Welcome to REM sleep. The rapid eye movement stage is so called because of the way your eyes move, well, rapidly. Although this might look random, usually the eye movement is a mirror of the way your eyes are moving in your dream.

And they’re the only part of your body that’s allowed to move. During REM sleep, the part of your brain that’s responsible for moving parts of your body shut down. Essentially, you’re paralysed.

Except, unlike paralysis, your heart rate goes up, your breathing turns irregular, and you can be sexually stimulated — and you are, several times a night. Even if your dreams aren’t sexual in their content.

So. Weird, huh?

For a long time, people believed that dreams were a deeply symbolic way for your brain to process your past or your feelings. A classic example is Sigmund Freud, who popularised the theory, along with Carl Jung. Freud said that dreams were your body’s way of revealing it’s deepest, most unconscious wishes and desires.

We’re now pretty sure this is not the case. Dreams are widely considered the result of thousands of neurons, firing at random inside your brain, with roots not quite as layered but emotionally significant all the same. There isn’t really a method to the madness, even though as humans, we look for it everywhere.

Speaking of madness, some people even liken dreaming to psychosis. Psychosis is a mental condition characterised by delusions and hallucinations. In a way, isn’t that what dreaming is?

Doesn’t hallucination sound like an apt word for believing it when you see a red zebra being chased by a blue lion across a purple field?

Biologically, there’s a reasonable explanation for why we believe dreams so easily. Firstly, control is disabled for the reasoning and impulse-control centres of the brain. And secondly, the secretion of some key communication chemicals are shut off.

Essentially, you might consider yourself to be watching a movie, in which you are the protagonist. Except that here, you’re also the director, screenwriter, and producer.

Sleeping is not a passive act. You might be sleeping like a log on the outside, but that’s only because it’s time for inside action to happen. You keep cycling between the four stages of sleep all through the night, repairing your body and refreshing your mind.

And then, you wake up.

(For real, this time. Not like the fake-out I did with REM sleep.)