Why we dream: what sleep science says
Everyone dreams every night, yet few can say what a dream is. Science still has no single answer, but decades of research have taught us a good deal.
Which part of sleep the dream is in
In 1953 researchers found that sleeping people's eyes now and then move rapidly, a stage that was named rapid eye movement sleep (REM). Wake people then, and most say they were having a vivid dream.
A night's sleep is four or five cycles of about ninety minutes, and REM grows longer towards morning. An adult spends roughly an hour or two in it each night, which is why the dreams near dawn tend to be the longest and strangest. Dreaming also happens outside REM, but it is usually shorter and more like "thinking something over".
How a night arranges its dreams
A night's sleep goes round in cycles: light sleep first, then deep sleep, then REM, then back to light sleep, and so on four or five times. The first half of the night has more deep sleep and the second half more REM, so in general the dreams just before dawn are longer, more vivid and more absurd.
This explains a familiar thing: someone who sleeps too little, especially who wakes far too early, loses much of their REM sleep, and when they finally catch up, the dreams come back suddenly many and clear. Researchers call it REM rebound. It is why the first good sleep after some late nights is full of vivid dreams.
Dreams in deep sleep are not absent, only more like scraps of thought and image, without the full story of REM dreams. Wake someone in deep sleep and they often remember no dream at all, which is not surprising.
What the brain is busy with in a dream
Sleeping does not switch the brain off. In REM sleep the amygdala, which handles emotion, is very active, while the prefrontal cortex, which handles logic and self-monitoring, is relatively quiet. That probably explains why dream feelings are vivid while the plot makes no sense. At the same time the body's muscles are held still, so lying there you cannot move, and the common dream experience of being unable to run or shout fits.
Many studies hold that sleep is when the brain sorts out the day: useful memories are consolidated and tied to what is already known, and feelings are cooled. A dream may well be the shadow that this sorting leaves in consciousness.
How the day gets into the dream
Dreams like to borrow from recent experience, above all things heavy with feeling and not yet dealt with. Freud called this "day residue". Experiments show something similar: people who play an absorbing game before bed sometimes dream of images from it that night, even when they did not feel they remembered it clearly while awake.
Interestingly, what turns up in a dream is often not only the day itself but something from five or six days earlier. Researchers call it the dream-lag effect: the brain sets a thing aside and lets it surface in a dream a few days later. So the people and events in a dream need not be from yesterday, and looking back over last week often pays off.
The body and its surroundings also slip into dreams: the alarm rings and becomes a bell in the dream; the room is too hot and the dream catches fire; the bladder is full and the dream hunts everywhere for a toilet. Such dreams are plain and need no deep reading.
How the night "washes" feeling
A widely discussed view holds that REM sleep is a kind of overnight tidying of emotion: the body's stress-related chemistry falls to its lowest, and the brain replays the day's emotional experiences and reprocesses them in a setting with no tension, so that on waking the same matter stings less.
This is still under study and not settled. But it fits what many people feel: a troubling matter often weighs less after a night's sleep. The old advice to sleep on it may have something in it.
Conversely, when sleep is broken or stress stays high, the tidying is left unfinished, the feeling stays where it was, and nightmares come more easily. That is also why people who sleep badly for long are often more fragile in their moods.
Several influential accounts
The activation-synthesis hypothesis (Hobson and McCarley, 1977) holds that a dream is random signals from the brainstem put together by the brain into a story, so the strangeness comes from the "putting together" and not from a hidden meaning. Later versions admit that what gets put together is your own memories and concerns.
The threat simulation hypothesis (Revonsuo, 2000) holds that a dream is a safe practice ground where one rehearses being chased, falling or failing, which is why nightmares hold more threats than daily life does.
The continuity hypothesis (Domhoff, Schredl and others), from large collections of dream reports, finds that dreams often continue what one cares about by day: what you care about is what you dream about.
What these accounts tell us
They do not contradict one another. They describe different sides of one thing: a dream is made of your own material, a kind of work the brain does in a quiet state, with memory in it, with emotion, and also a rehearsal of what may come.
Science does not support the idea that dreams foretell the future. But dreams do often reveal what you really care about lately, and that is the value of reading them: not to read fate but to read yourself.
Questions people ask
Some people say they never dream. Is that true?
Almost everyone dreams; they just do not remember it on waking. Dream memory is fragile and fades within minutes. To remember dreams, see the article on how to remember them.
Why is the plot of a dream so strange?
The prefrontal cortex, which reasons and checks, is less active in dreams, so the brain puts memory fragments together at random and feels nothing wrong, until you wake and see the absurdity.
Does it matter if I dream a lot and sleep badly?
Dreaming a lot now and then is common. If you sleep badly for a long time or are clearly sleepy by day, please see a doctor. Sleep problems can be helped.
Why do some people remember dreams every day and others almost never?
Research finds that people who often recall dreams tend to wake more often in the night, and in those few minutes the dream has not yet faded; they are also more interested in dreams, and the brain areas for attention and memory are more active in their sleep. It is not a matter of better or worse, only of different ways of sleeping. To remember more, see the article on how to remember dreams.