What Happens in the Brain When We Dream?


Özge Üstündağ

Have you ever had a dream so vivid that you thought it was real? Sometimes, dreams resemble so much of our real-life experience that we wish they would last forever. Especially if they involve a trip to Bali!

While some people feel deeply affected by their dreams, there are others who do not think that they even dream at all. However, sleep researchers have argued that we all dream even if we do not remember what we see. The main reason behind this idea lies in the five stages of sleep and the type of brain activity associated with them.

So, what happens in this mysterious dream place we enter every night?

A closer look at sleep
In 1953, Nathaniel Kleitman and Eugene Aserinksy showed via EEG (electroencephalograph) recordings that sleep occurs in stages. There are two main types of sleep: REM sleep which literally means rapid-eye-movement and NREM which is made up of four stages.
NREM sleep Stage 1: We feel drowsy; pulse slows down and our muscles relax.
Stage 2: As we enter light sleep, our body temperature drops. Here, the frequency of EEG waves decreases (waves slow down) while their amplitude (height of a wave) increases. In this state, we also experience something called sleep spindles. Brain waves rapidly increase and decrease for a few seconds.
Stage 3: We enter deep sleep; the intermittent spikes in brain waves decrease.
Stage 4: Brain waves oscillate at a very low frequency (1 – 4 Hz) and they start to get higher in amplitude; long delta waves are observed.
As a person moves into a deeper state of sleep, it gets progressively harder to wake them up. You know what we are talking about if you tried to wake someone up in the middle of night!
Then something different happens following stage IV. We enter REM sleep: a state in which our eyes move rapidly and our muscles become atonic. This stage is also called paradoxical sleep because certain areas of the brain light up as if we are awake. 

Dreams and REM

Up until recently, researchers thought that dream experiences occurred only in REM sleep. However, latest scientific evidence suggests that dreams can also happen in NREM stages. In fact, people have reported dream experiences in NREM sleep up to 70% of the time.

In a study published in Nature Neuroscience, Francesca Siclari and colleagues confirmed that dreams can occur during any sleep stage. For this study, the researchers investigated both the dreaming experiences of participants and dream content. Regarding dream content, the researchers asked study subjects to rate their dreams on a scale of thought-like (thinking and/or reasoning) to perceptual (vivid images, no reasoning). Moreover, they also analyzed whether dreams contained faces, movement or speech.

Siclari and her colleagues found that there is a “hot zone” region in the brain associated with dreaming. When the parieto-occipital region (a region towards the back of the brain) showed an increase in low-frequency (slow) EEG activity, study participants reported no dreams. However, subjects reported dreams when this hot zone showed a decrease in low-frequency EEG activity. The researchers report that they observed this finding regardless of the type of sleep (REM or NREM) participants were in.

The investigators of the study also found an interesting association between subjects’ dream content and waking perception of the same contents. These findings suggest that dream content (thoughts, perceptions, faces, places, movement and speech) is associated with an increase in fast EEG activity in specific brain areas which resemble activity in awake state.

The researchers suggest that these findings “[provide] further evidence that dream reports reflect conscious experience during sleep rather than confabulations upon awakening.” In other words, what we remember from our dreams are not mere fabrications to fill in the gaps in our memories but reflections of real (neural) experiences. 

REFERENCES

  • 1. Siclari, F., Baird, B., Perogamvros, L., Bernardi, G., LaRocque, J. J., Riedner, B., Boly, M., Postle, B. R., & Tononi, G. (2017). The neural correlates of dreaming. Nature Neuroscience, 20, 872-878. doi:10.1038/nn.4545
  • 2. Stickgold, R., Malia, A., Fosse, R., Propper, R., Hobson, J. A. (2001). Brain-mind states: I. Longitudinal field study of sleep/wake factors influencing mentation report length. Sleep, 24, 171–179. [PubMed]
  • 3. https://www.ncbi.nlm.nih.gov/books/NBK10996/