Why do we dream? What sleep science says

We dream every night, mostly during REM sleep, and research still hesitates between several explanations for this activity.

REM sleep
Continuity
Hypotheses

The essentials

Why do we dream? at a glance

For you, remember above all that your dreams often pick up your days: it is the idea best supported by studies.

Free tool: Interpret my dream
Also read: Remembering your dreams

Interpret my dream

When do we dream?

In 1953, Eugene Aserinsky and Nathaniel Kleitman described phases of sleep in which the eyes move rapidly under the eyelids: REM sleep. Woken at that moment, people most often report a dream. An adult goes through four or five of these phases a night, longer toward morning; they take up about a quarter of sleep. So we dream several times a night, even with no memory on waking.

We dream outside it too

REM sleep does not hold a monopoly on dreaming. Woken during the other phases, people also describe scenes, often shorter and less strange. In 2017, a team led by Francesca Siclari and Giulio Tononi tied dreaming, in both types of sleep, to a drop in slow activity in an area at the back of the brain. The result, obtained on a small group, remains debated but got a lot of attention.

Hypotheses about its function

No one yet knows with certainty what dreaming is for. The continuity hypothesis, carried by G. William Domhoff, notes that our nights mostly replay the worries and scenes of our days. The threat simulation hypothesis, proposed by Antti Revonsuo, sees in it a rehearsal for dangers; it is debated. Other work links dreaming to the sorting of memories. These leads are not mutually exclusive, and none makes the dream a forecast.

Facts and sources

Eugene Aserinsky and Nathaniel Kleitman described in 1953 rapid eye movement sleep (REM sleep), from which awakenings yield the most dream reports. E. Aserinsky and N. Kleitman, “Regularly occurring periods of eye motility, and concomitant phenomena, during sleep,” Science, 118, 1953, pp. 273–274
An adult goes through four or five periods of REM sleep per night, longer toward morning; they take up about 20 to 25% of sleep. Wikipedia (en), “Rapid eye movement sleep”
People also dream outside REM sleep: in 2017, a team linked dreaming, in both types of sleep, to a decrease in slow waves in a posterior area of the brain. F. Siclari et al., “The neural correlates of dreaming,” Nature Neuroscience, 20, 2017, pp. 872–878
Several hypotheses coexist without a verdict: continuity with waking life (Domhoff, 1996), threat simulation (Revonsuo, 2000, debated). G. W. Domhoff, Finding Meaning in Dreams, 1996; A. Revonsuo, Behavioral and Brain Sciences, 23 (6), 2000

A dream is read as a mirror of what you are living, never as an omen: no reading here predicts anything at all.

Why do we dream: your questions

How many dreams do we have per night?

Several: each phase of REM sleep, four or five a night in adults, is often accompanied by a dream, and others arise during the rest of sleep. Most are forgotten. We generally keep only the last one, the one that precedes waking.

Do animals dream?

Many mammals and birds have REM sleep, and some behaviors in it suggest dreams. But a dream is known through an account, and animals tell nothing: the question remains open.