Dream science
Why do we dream?
Nobody knows for sure why we dream (NINDS, 2025). Scientists have several leading ideas: dreams may rehearse danger, act out what’s on your mind, turn brain activity in sleep into a story, or help the brain sort memories and feelings (Nir & Tononi, 2010; Sterpenich et al., 2020). Each has some evidence and real gaps, and they can overlap.
Last updated: 2026-10-13.
Do scientists know why we dream?
Scientists don’t yet know why we dream. The US National Institute of Neurological Disorders and Stroke (NINDS, 2025) says the exact purpose of dreaming isn’t known, though dreaming may help you process your emotions.
Dreams are hard to study: researchers can only collect what people report after they wake, and they can’t easily control what someone will dream in an experiment (Nir & Tononi, 2010).
Are dreams a rehearsal for danger?
Dreams may rehearse danger, but the evidence for this idea, the threat simulation theory, is mixed (Zadra et al., 2006). Revonsuo (2000) proposed that dreaming evolved to simulate threats, so the brain could practice spotting danger and getting away from it. Early humans, he reasoned, lived short lives full of danger, so a mind that rehearsed threats at night would have given them a better chance of surviving to have children.
Some findings fit the theory: severely traumatized Kurdish children reported more dreams, with more threats and more severe ones, than less traumatized Kurdish children or Finnish children with no trauma (Valli et al., 2005). In 212 recurring dreams, 66% held at least one threat, and the threats tended to be dangerous and aimed at the dreamer (Zadra et al., 2006).
Other findings don’t fit: fewer than 15% of those recurring dreams showed a realistic situation critical to survival or reproduction, and the dreamers rarely got away. Zadra et al. (2006) called the support mixed.
Malcolm-Smith and Solms (2004) collected the most recent dream of each of 401 people, and only 8.48% of them had dreamed of a realistic threat to their life. A realistic escape followed in only a third of those dreams. Yet 44.58% of the same people had faced a severe threat to their life while awake, and the authors concluded that these findings contradict key parts of the theory. For frightening dreams of your own, see the guide to nightmares and when to get help.
Do dreams come from your waking life?
Dreams draw on waking life much of the time, but they rarely replay it (Fosse et al., 2003). Researchers call this link the continuity hypothesis (Schredl & Hofmann, 2003). In Domhoff’s (2017) account, dreams very often act out the personal concerns that run through your waking thoughts. He argues that the strength of those concerns, not the events of the day, shapes the core of what you dream. In the studies he reviews, dreamers can identify even a slight trace of the day before in only about half of their dreams.
In a diary study, the more time people spent on an activity, the more they tended to dream about it (Schredl & Hofmann, 2003). Focused work such as reading and writing seldom appeared, though, so the authors say the hypothesis needs to be stated more precisely.
When 29 people logged their days and their dreams for 14 days, 65% of 299 dream reports reflected something from recent waking life, but no more than 1 to 2% replayed an actual episode (Fosse et al., 2003).
Are dreams the brain making a story out of random signals?
The idea that dreams are stories spun from signals in the sleeping brain comes from the activation-synthesis hypothesis, which some later findings don’t fit well (Solms, 2000). Hobson and McCarley (1977) proposed that a mechanism in the brainstem produces dreaming sleep, and that higher brain areas, once switched on, make up a dream by comparing those signals with stored memories.
The brainstem plays a special role in REM (rapid eye movement) sleep, the stage when your eyes move quickly behind closed lids and dreams are usually most vivid (NINDS, 2025). Yet Solms (2000) reviewed evidence that in some patients with damage to higher brain areas, dreaming stops while REM sleep carries on. Dreaming can also start in non-REM sleep, the other main kind of sleep (NINDS, 2025), as the guide to dreams outside REM sleep shows. Solms (2000) concluded that the brainstem’s REM mechanism is only one of many triggers for a separate dreaming system higher in the brain.
Do dreams help with memory and emotions?
Dreaming may play a part in memory and emotion, though the studies so far can’t show that a dream itself does the work (Sterpenich et al., 2020).
In a memory study, 99 people learned their way around a virtual maze, and 50 of them then took a nap (Wamsley et al., 2010). Four of the nappers mentioned the maze when they described their dreams. At the retest, those four had improved tenfold more than the nappers who didn’t, while thinking about the maze when awake didn’t predict any gain. The authors don’t claim that the dreams caused the improvement; they suggest that dreaming may reflect memory processing in the sleeping brain. It’s a correlation, and it rests on four people.
Sterpenich et al. (2020) tested one version of the emotion idea: that fear in dreams goes with better-adjusted reactions to threats when you’re awake. When 18 people were woken several times through the night, dreams that held fear went with more activity in two brain areas that also respond to unpleasant sights when you’re awake. In a separate study, the same team found that people who reported more fear in their dreams tended to react somewhat less to frightening pictures while awake, both in brain scans (89 people) and in pupil size (77 people). That fits the idea, but it’s only a link, found in healthy volunteers. The authors suggest recurring nightmares may not work this way.
Which theory has the best evidence?
No theory of dreaming has won yet: each explains some findings and misses others.
| Theory | The idea | Evidence for | Where it falls short |
|---|---|---|---|
| Threat rehearsal | Dreams let the brain practice facing danger (Revonsuo, 2000) | Severely traumatized children dreamed of more and worse threats than other children (Valli et al., 2005); 66% of 212 recurring dreams held a threat (Zadra et al., 2006) | Under 15% showed a realistic threat to survival (Zadra et al., 2006); 8.48% of 401 people had a realistic deadly threat in their latest dream (Malcolm-Smith & Solms, 2004) |
| Continuity | Dreams act out the concerns on your mind (Domhoff, 2017) | 65% of 299 dream reports reflected recent waking life (Fosse et al., 2003); time spent on activities shows up in dreams (Schredl & Hofmann, 2003) | Describes what dreams contain without saying why we have them (Nir & Tononi, 2010) |
| Activation-synthesis | Higher brain areas make a story from brainstem signals in sleep (Hobson & McCarley, 1977) | Brain research of the time tied dreaming sleep to a brainstem mechanism (Hobson & McCarley, 1977) | In some patients with damage to higher brain areas, dreaming stops while REM sleep goes on, and dreams also start outside REM (Solms, 2000) |
| Memory and emotion | Dreaming may help process emotions (NINDS, 2025) and track how sleep handles memories (Wamsley et al., 2010) | Maze dreamers improved tenfold more than other nappers (Wamsley et al., 2010); more fear in dreams went with somewhat calmer fear responses awake (Sterpenich et al., 2020) | The studies show links only, and the maze finding rests on 4 people (Wamsley et al., 2010) |
What don’t scientists know yet?
Several basic questions about dreaming are still open:
- Whether dreaming has a purpose at all. In the view of Foulkes and Domhoff, as Nir and Tononi (2010) summarize it, dreaming probably has no function of its own and is a by-product of how sleep and consciousness evolved.
- Whether the experience of a dream matters, or only mirrors work the brain does in sleep anyway. Wamsley et al. (2010), who ran the maze study, lean toward the second.
- What reports leave out. You may not remember most of your dreams (NINDS, 2025), for reasons set out in why most dreams fade.
To study your own dreams, start by trying to remember more of them.
More questions about why we dream
Do people who were born blind dream?
People born blind do dream, though blindness changes what their dreams are made of (Meaidi et al., 2014). In the authors’ four-week diary study, 11 people blind from birth and 14 who lost their sight later had fewer visual dream impressions than 25 sighted people. Those blind from birth also reported more sound, touch, taste and smell than the sighted group. A smaller study of 10 people blind from birth and 9 sighted people concluded that the blind group’s dreams do have visual content (Bértolo et al., 2003), so how much is visual has been debated (Nir & Tononi, 2010).
If dreams have a purpose, does that tell you what yours means?
A purpose for dreaming wouldn’t tell you what any one dream means. The theories describe what dreaming does for people in general, and none offers a key to a single dream. Dreams tend to follow the dreamer’s own concerns (Domhoff, 2017), so your own life is the place to start, as the guide to whether dreams mean anything explains.
Why do recurring dreams so often involve a threat?
No one knows for sure why recurring dreams so often hold a threat (Zadra et al., 2006). In the 212 recurring dreams they studied, 66% contained at least one, and when threatened, dreamers usually defended themselves or tried to escape in sensible ways. Threat rehearsal reads that as practice, though under 15% showed a realistic threat to survival. Being chased is one such threat, covered in dreams of being chased, and the guide to dreams that repeat looks at why some come back.
Your own dreams over time
If you want to watch the continuity idea play out in your own nights, start a journal. Dreams Come True, a voice-first dream journal app for iPhone, keeps yours on your phone. With Pro, it shows the symbols that recur over the weeks.
Sources
- Bértolo, H., Paiva, T., Pessoa, L., Mestre, T., Marques, R., & Santos, R. (2003). Visual dream content, graphical representation and EEG alpha activity in congenitally blind subjects. Cognitive Brain Research, 15(3), 277–284. doi.org/10.1016/S0926-6410(02)00199-4
- Domhoff, G. W. (2017). The invasion of the concept snatchers: The origins, distortions, and future of the continuity hypothesis. Dreaming, 27(1), 14–39. doi.org/10.1037/drm0000047
- Fosse, M. J., Fosse, R., Hobson, J. A., & Stickgold, R. J. (2003). Dreaming and episodic memory: A functional dissociation? Journal of Cognitive Neuroscience, 15(1), 1–9. doi.org/10.1162/089892903321107774
- Hobson, J. A., & McCarley, R. W. (1977). The brain as a dream state generator: An activation-synthesis hypothesis of the dream process. American Journal of Psychiatry, 134(12), 1335–1348. doi.org/10.1176/ajp.134.12.1335
- Malcolm-Smith, S., & Solms, M. (2004). Incidence of threat in dreams: A response to Revonsuo’s threat simulation theory. Dreaming, 14(4), 220–229. doi.org/10.1037/1053-0797.14.4.220
- Meaidi, A., Jennum, P., Ptito, M., & Kupers, R. (2014). The sensory construction of dreams and nightmare frequency in congenitally blind and late blind individuals. Sleep Medicine, 15(5), 586–595. doi.org/10.1016/j.sleep.2013.12.008
- National Institute of Neurological Disorders and Stroke. (2025, February 25). Brain basics: Understanding sleep. National Institutes of Health. ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep
- Nir, Y., & Tononi, G. (2010). Dreaming and the brain: From phenomenology to neurophysiology. Trends in Cognitive Sciences, 14(2), 88–100. doi.org/10.1016/j.tics.2009.12.001
- Revonsuo, A. (2000). The reinterpretation of dreams: An evolutionary hypothesis of the function of dreaming. Behavioral and Brain Sciences, 23(6), 877–901. doi.org/10.1017/S0140525X00004015
- Schredl, M., & Hofmann, F. (2003). Continuity between waking activities and dream activities. Consciousness and Cognition, 12(2), 298–308. doi.org/10.1016/S1053-8100(02)00072-7
- Solms, M. (2000). Dreaming and REM sleep are controlled by different brain mechanisms. Behavioral and Brain Sciences, 23(6), 843–850. doi.org/10.1017/S0140525X00003988
- Sterpenich, V., Perogamvros, L., Tononi, G., & Schwartz, S. (2020). Fear in dreams and in wakefulness: Evidence for day/night affective homeostasis. Human Brain Mapping, 41(3), 840–850. doi.org/10.1002/hbm.24843
- Valli, K., Revonsuo, A., Pälkäs, O., Ismail, K. H., Ali, K. J., & Punamäki, R.-L. (2005). The threat simulation theory of the evolutionary function of dreaming: Evidence from dreams of traumatized children. Consciousness and Cognition, 14(1), 188–218. doi.org/10.1016/S1053-8100(03)00019-9
- Wamsley, E. J., Tucker, M., Payne, J. D., Benavides, J. A., & Stickgold, R. (2010). Dreaming of a learning task is associated with enhanced sleep-dependent memory consolidation. Current Biology, 20(9), 850–855. doi.org/10.1016/j.cub.2010.03.027
- Zadra, A., Desjardins, S., & Marcotte, É. (2006). Evolutionary function of dreams: A test of the threat simulation theory in recurrent dreams. Consciousness and Cognition, 15(2), 450–463. doi.org/10.1016/j.concog.2005.02.002