Why do we dream? The theories, what's known, and what's still open

A dream is a mental experience, with images, emotions and sometimes a story, that happens while we sleep. According to the Sleep Foundation, a person dreams about two hours a night on average, though almost all of it is forgotten on waking.

We dream in every stage of sleep, but dreams are most abundant and intense in REM sleep, when brain activity rises sharply. REM dreams tend to be more vivid, fantastical or strange; non-REM dreams tend to be more coherent and tied to a specific place and time, like a thought that carries on. And because REM is concentrated in the second half of the night, much of what we remember was dreamed in the last hours before waking.

The Sleep Foundation sums up the state of play: the debate is still open, and it may be impossible to prove any single theory conclusively. These are the five ideas that come up most:

• Consolidating memory. Sleep strengthens what we learned, and dreaming would be part of that work.
• Processing emotions. Dreaming would let us rehearse feelings in an imaginary setting with no real consequences, and so handle them better.
• Tidying the mind. The brain would discard partial, wrong or unnecessary information gathered during the day.
• Replaying experience. Dreaming would be a distorted version of reviewing recent events.
• Being a by-product. Dreaming would simply be the brain's activity during sleep, with no function of its own.

They aren't mutually exclusive. The same dream can be a replay of your day, a rehearsed emotion and a bit of noise, all at once.

Beyond that general picture, three specific proposals are cited often in the scientific literature.

Activation-synthesis. In 1977, Hobson and McCarley proposed that dreaming arises from the brain's activation during REM, and that the mind "synthesises" a story to make sense of that activity. It took some of the mystery out of dreaming, and it is still debated and refined today.

Threat simulation. In 2000, Antti Revonsuo proposed that dreaming has an evolutionary function: rehearsing dangerous situations and how to avoid them. It would fit threats being common in dreams. Studies that have tested it, however, find partial support, not conclusive.

Sleep and emotion. In 2009, Walker and van der Helm reviewed the research on sleep and the emotional brain. Their conclusion is that REM sleep plays a role in how emotionally charged memories are processed, an argument for the idea that dreaming and emotion regulation are connected.

The Sleep Foundation puts it in one phrase: dreams can be forgotten in the blink of an eye. Many people notice that what seemed crystal clear on waking vanishes in the shower. There's no definitive explanation of the mechanism, but there are practical habits for remembering more:

• Think about the dream as soon as you wake, before sitting up or talking to anyone.
• Keep pen and paper (or your phone) within reach, and note it straight away, even if you wake in the small hours.
• Wake gently if you can: a harsh alarm yanks you out of the dream and takes the details with it.
• Before sleeping, remind yourself you want to remember. It sounds small, but it helps you reflect on waking.

More on this in our guide to the dream journal.

That nobody knows what they're for doesn't stop them being useful. Dreams tend to gather what worries you, excites you or occupies your head, using images that are yours: people you know, places you've been, fears you already had. That's why the same image means different things to different people.

What they don't tell you: the future, what someone else thinks of you, or the state of your health. When we dream of losing teeth or being chased, there's no hidden message from the universe: there's an experience common to a great many people, expressed in personal images.

That's why at Onirah we treat every dream as a starting point for a question, not a code to crack. To see how, start with the interpretation guide, or try symbols that recur across many cultures: falling, being chased or teeth falling out.

You don't need to wait for science to settle the debate to get something from a dream. Three practical attitudes hold whichever theory turns out right.

• Treat it as a hypothesis. If a dream suggests something, test it against your life: does it fit something you already felt? If not, drop it without drama.
• Look at the present, not the future. All the theories above are about memory, emotion or current brain activity. None claims dreaming exists to anticipate what will happen.
• Watch the emotion. If emotional processing has anything to do with it, what you felt in the dream is the most reliable datum you have, more than the plot.

With that, any dream can be a starting point for a simple question: which matter in my waking life resembles this?

It's worth closing with honesty. We don't yet know whether dreaming has a function of its own or is a consequence of other processes. We don't know why some dreams carry enormous emotional charge and others are banal. And we don't know why some people recall their dreams almost every morning and others almost never. Research is moving, but any claim of the form "dreams are for X" presented without nuance should be taken with caution.

Given that uncertainty, a reasonable stance is a practical one: the ultimate cause matters less if the dream hands you an image to think with. Use it as material for reflection, not as proof. If you have very disturbing dreams that keep coming back, we cover them in the nightmares guide.