Humans spend a third of a lifetime sleeping or trying to fall asleep. Sleep deprivation can result in a variety of cognitive deficits, such as depressed mood, speech disfluency, reduced alertness, attentiveness, and ability to learn or remember things. Sleep is essential to our physiological and psychological well-being, as it allows us to recharge overnight in order to face a brand new day with our cognitive abilities intact. So, it seems that sleeping is necessary. But is dreaming? What even is a dream? And more importantly, what happens to our brain when we dream?
A dream can be defined as “subjective experience during sleep” that is solely accessible by the dreamer after waking up. Everybody dreams, even if they do not remember having done so. In an average 8-hour-long sleep, we dream for approximately 2 hours! Dreaming mostly occurs in rapid-eye movement (REM) sleep, but to a limited degree, can also take place in non-REM sleep. During REM sleep, arm and leg muscles become temporarily paralyzed, while the eyes move quickly from side-to-side, and breathing, heartrate, and brain activity increase to nearly-awake levels. These features have led researchers to characterize REM sleep as a “metabolically awake brain in a paralyzed body.”
Our dreams are not detached from neural activity, such that they exist in a vacuum – in fact, they very much reflect the inner workings of the brain. The perceptual modalities that dominate our wakeful state are also present in most dreams; consider how dreams tend to be vivid with colour, shape and movement, and contain categories we are well acquainted with, such as people, places, or animals. Dreams contain sounds, like speech or conversation, and sometimes, can include tactile perceptions such as pleasure, pain, or smell and taste. Evidently, experiences within dreams are not alien abstractions void of sensory substance, but are “seen, heard and felt.”
These perceptual similarities between sleep and wakefulness are reflected in neurophysiological similarities as well. Electroencephalogram (EEG), which monitors electrical activity in the brain, reveals similar states in the active waking brain and REM sleep. Positron emission tomography (PET) has shown that global brain metabolism in REM sleep is comparable to that of being awake. There is also consistency between the cognitive and neural organization of dreaming and being awake – for example, people with impaired face perception do not dream of faces. And the features of children’s dreams parallel that of their waking cognitive development.
In examining the neuroanatomy of REM sleep, researchers have been able to track the relative increase and decreases in neuronal activity. It is quite fascinating that, paralleling vivid visual imagery while dreaming, the visual cortex – which is responsible for processing visual information – becomes highly activated.
Most of the time, we lack voluntary control in dreams; things just happen, and we carry along with the events that unfold. This could be a function of the right inferior parietal cortex becoming deactivated during REM sleep, a region of the brain which has recently been associated with free will (or waking volition). We also experience altered reflective thought, such that, we accept absurd events as true – for example, swinging from the moon, or the impossible jarring transitions from one dream-scene to the next. In dreams, we often feel uncertain about time and space or the identities of the characters that come and go. This could be a consequence of deactivation in a variety of brain regions (such as the dorsolateral prefrontal cortex) during REM sleep. In fact, deactivation of the prefrontal cortex is associated with reduced self-awareness when engaging with highly sensory experiences while awake.
Some of our dreams are emotional, we feel the things we dream up, whether this is fear, anxiety, or joy. Unsurprisingly then, REM sleep is associated with increased activation of the amygdala, anterior cingulate cortex and insula; these brain structures are implicated in emotion and emotion regulation, and the fight or flight response. However, in approximately 25% of REM sleep, emotion is absent – even in the types of situations we would typically feel something should they be encountered while awake. And little do we experience low moods (such as sadness or guilt), which could be an effect of the reduced self-awareness.
We don’t always remember our dreams after waking up; dream recall tends to be best following REM awakening and can vary within and between people. Factors such as personality, sleep duration, and visual memory have been associated with dream recall. Several studies even point to sex differences, finding that women recall their dreams more often than men.
Some researchers suggest that just as we need to sleep, we also need to dream. They forward that dreaming has a healing quality, and reduces the sting of painful waking emotional experiences; that dreaming provides a safe environment to re-process upsetting memories and reach emotional resolutions when we wake up the next day. Maybe then, an evolutionary function of dreaming is overnight self-therapy.