Memories, dreams, and simulations: What does the brain do when we are asleep?

'The Dream' by Henri Rousseau.

Image credit: Henri Rousseau/Public domain.

First published on 17th November 2013. Last updated on 5th August 2017 by Dr Helen Klus

1. Sleep

1.1 What is sleep?

We spend about a third of our lives sleeping, and much of that time dreaming, yet we still don't know why this happens.

Almost all animals are thought to sleep. In complex animals, sleep can be defined as a state of reversible unconsciousness, illustrated by a change in brain wave patterns and eye movements[1a]. In simpler animals, like invertebrates, sleep can be defined as a state where the creature periodically stops responding to external stimuli. The simpler the creature, the more difficult it is for researchers to determine whether or not they have fallen asleep[2].

The most primitive creature thought to sleep is the Caenorhabditis elegans, a type of roundworm that becomes unresponsive before it moults, indicating that sleep may be connected to physical development[3].

More complex invertebrates are thought to sleep, although this is difficult to prove. Reptiles sleep, and this can be shown from their brain wave patterns, which differ from those of sleeping birds and mammals[4]. Birds and mammals also periodically undergo a different stage of sleep to other animals, known as rapid eye movement (REM)[5a]. This is the stage of sleep in which we're most likely to dream[6].

Different species of mammals sleep for different durations, and members of the same species have been shown to sleep longer when in captivity[7]. Carnivores tend to sleep for longer than omnivores, and herbivores tend to sleep the least, with smaller herbivores often sleeping for longer than larger ones[5b]. The state that an animal enters when hibernating is different from sleep, and so hibernating animals also sleep periodically[8].

Some birds and aquatic mammals like eared seals, whales, and dolphins can exhibit unihemispheric sleep[9]. This means that one side of their brain can sleep while the other is awake. When the right side is asleep, the left eye is shut, and vice versa. This allows birds to continue to fly while sleeping, and allows aquatic mammals to swim to the surface to breathe. Evidence of unihemispheric sleep has also been found in cats that have had the connection between their left and right hemispheres severed[10].

REM sleep does not occur during unihemispheric sleep, but many animals that exhibit unihemispheric sleep will start to sleep with both hemispheres when possible - such as when they move to land or enter captivity - and will then exhibit REM sleep[11].

1.2 What happens to our brain when we sleep?

We can determine some of what happens to us during sleep by looking at brain wave patterns. Brain waves are waves of electricity emanating from neurons in the brain[12a].

Neurons are cells that can gather and transmit information via electrical signals, along a neural pathway. There are three types of neurons: sensory neurons, motor neurons, and interneurons.

Sensory neurons carry signals from the outer parts of the body to the central nervous system, and motor neurons do the opposite. Interneurons carry information between motor and sensory neurons.

Diagram showing three types of neurons: interneurons, which are bipolar, sensory neurons, which are unipolar, and motor neurons, which are multipolar.

The three basic types of neuronal arrangements. Image credit: Holly Fischer/CC-A.

Diagram of a neuron cell.

Complete neuron cell diagram. Image credit: LadyofHats/Public domain.

The simplest type of neural pathway does not involve the brain. In a knee-jerk reaction, for example, when a person's knee is taped in the right place a sensory neuron sends a signal to a motor neuron in their central nervous system, which sends a signal to make a muscle in their leg contract. The brain only becomes involved in more complex situations[12b].

There are about 100 billion neurons in the human brain, and the amount of electronic signals they emit depends on how much stimulus they are receiving[13]. Neurons emit more electronic signals per second, and therefore have a higher electronic frequency, when they receive more stimuli. This frequency can be measured by placing electrodes on the top of a person, or animal's, head, in what is known as 'electroencephalography' (EEG).

Brain wave patterns can be divided into five types[1b]:

  • Gamma brain waves have the highest frequency, and are associated with states when we are highly alert, such as when we are afraid, or when we are concentrating on a complex task.

  • Beta brain waves have a slightly lower frequency than gamma brain waves, and are associated with less complex tasks.

  • Alpha brain waves have an even lower frequency, and occur when we are relaxed, and during REM sleep.

  • Theta brain waves have a lower frequency than alpha brain waves, and also occur during REM sleep.

  • Delta brain waves have the lowest frequency, occurring during deep sleep or unconsciousness, although young children and babies can exhibit delta waves while they are awake[14].

Humans undergo five stages of sleep, which repeat every 90 minutes or so. These are categorised by different brain wave patterns, different muscle movements, which are measured with an electromyogram (EMG) machine, and different eye movements, which are measured with an electrooculogram (EOG). Heart rate, breathing rate, and blood pressure can also be measured during sleep[1c].

Diagram showing the five stages of sleep. Image states:‘Stage 1, 4-5%. Light sleep, muscle activity slows down. Occasional muscle twitching. Stage 2, 45-55%. Breathing pattern and heart rate slows. Slight decrease in body temperature. Stage 3, 4-6%. Deep sleep begins. Brain begins to generate slow delta waves.Stage 4, 12-15%. Very deep sleep. Rhythmic breathing. Limited muscle activity. Brain produces delta waves. Stage 5, 20-25%. Rapid eye movement. Brainwaves speed up and dreaming occurs. Muscles relax and heart rate increases. Breathing is rapid and shallow.

The sleep cycle. Image credit: Sleep 1102/CC-NC-SA.

The first stage of sleep consists of very light sleep, as brain waves slow from alpha to theta waves, during this stage you can experience sudden muscle spasms, known as hypnic jerks, which can make you feel as if you are falling, you may also experience dream-like images or sounds[1d].

In the second stage of sleep, theta brain waves can exhibit spindles, clusters of rapid waves that are often followed by muscle movement, and K-complexes[1e]. K-complexes are single large waves, which may occur when the brain decides that it's safe to ignore external stimulus. They may also be associated with processing memories[15].

The third stage of sleep is deeper than the second stage, and brain waves slow to become delta waves, while stage four sleep is deeper still. Sleepwalking, sleep talking, and night terrors typically occur during stages three and four[16].

The fifth stage of sleep consists of REM sleep, which was discovered by physiologists Eugene Aserinsky and Nathaniel Kleitman in 1953[17]. When a person undergoes REM sleep, many of their muscles are paralysed and their eyes move rapidly under their closed eyelids, their heart rate and breathing rate increase, their blood pressure rises, and their brain activity increases to the same level as when they are awake[18].

Dreams mostly occur during periods of REM sleep, but have also been found to occur at any stage. As we repeat the five stages of sleep, we experience longer periods of REM sleep and shorter periods of deep sleep[1f].

No one knows why we experience REM sleep. One possibility is that it's important for brain development, providing the neural stimulation needed to develop the nervous system. This is known as the ontogenetic hypothesis. Evidence for this comes from the fact that we experience less REM sleep as we get older. If the ontogenetic hypothesis is true, then it would mean that REM sleep has no function in adults[19].

1.3 Why do we sleep?

Sleep appears to be vital for survival. This has been illustrated in laboratory experiments on rats[20], and in people suffering from fatal familial insomnia (FFI)[21]. FFI is a very rare condition that causes increasingly worsening insomnia, leading to hallucinations, and eventually organ failure. We still don't know exactly why we need to sleep, but there is evidence that sleep can aid in physical development, in memory recall[22], and in improving the immune system[23].

2. Dreams

2.1 What are dreams?

While dreaming, we see images without external stimulus, in a similar way to how we can imagine visual images that do not really exist. Dreams differ from these images, however, because when we are dreaming we are not actively in control of what we experience, and we tend to be unaware that the images we see in dreams are not real. This may be because we are unable to imagine any other type of reality than the one presented to us while we are dreaming[24].

The fact that we do not often realise when we are dreaming, even though there are many clues that the dream is not real, led philosophers such as Zhuang Zhou[25] and Rene Descartes to claim that we cannot trust our ability to distinguish reality from illusion[26].

Dream-like imagery. Credit: The Experience of Fliehkraft from Till Nowak on Vimeo.

People have recorded their dreams for at least 5000 years, with the earliest evidence coming from the Sumerians in Mesopotamia[27]. In the late 1800s and early 1900s, psychologist Sigmund Freud began one of the first efforts to systematically study dreams[28]. Freud believed that dreams occur so that we can fulfil wishes that we may not want to admit we have in our waking life. Psychiatrist Carl Jung expanded on this by stating that dreams have messages the dreamer should act upon[29]. Neither of these theories are currently accepted.

Scientific studies were made in the 1900s, when dreams became the subject of neurology. We now think that everyone dreams several times a night for about 2 hours in total. Individual dreams last about 5 to 20 minutes, with the period spent dreaming, and the vividness of dreams, increasing the longer we sleep[30]. Yet we still don't know what dreams are or why they occur.

One reason why it's so difficult to study dreams using the scientific method is that they are inherently subjective experiences. This is also true for many experiences we have when we are awake, our perspective of colour is subjective, for example. There's no way to know that the wavelength of light that you experience as the colour red, does not look blue to others, or for that matter, that anyone but you is really conscious at all. We have still not explained the nature of waking consciousness, and the nature of our consciousness while we dream is even more elusive[31].

The scientific study of dreams is also complicated by the fact that people find it much more difficult to remember their dreams than their waking life. Dreams can have continuously changing visuals that blend together and do not necessarily make sense, and people have the tendency to create narrative in random images[32]. There's also a bias where more intense or unusual dreams are easier to remember[33].

The memories of dreams can be triggered if similar events occur when we're awake, particularly if they're unusual. This can lead people to think they've had a dream that predicted the future. Although there's no evidence that this is more than a coincidence[34]. The fact that we find it difficult to remember our dreams suggests that there's no evolutionary advantage to remembering them. There are people who claim to never remember dreaming, and so the ability to remember our dreams is apparently not vital for survival.

Commonly reported dreams include sexual dreams and nightmares. Negative emotions are more commonly reported than positive emotions, with anxiety being the most common emotion[35]. Some people only recall dreams in black and white, and this has been linked to watching black and white television and films as children[36].

Some people experience lucid dreams of varying complexity. Lucid dreams occur when a person obtains some kind of self-awareness while they are dreaming. Some people can then control certain aspects of their dreams, or force themselves awake. Although the brain will often find a way to adapt to this, preventing the dreamer from awakening or providing a false awakening, where the dreamer dreams they have awoke[37].

Lucid dreaming allows dreamers to communicate with researchers, if they can, for example, deliberately move their eyes while they are asleep. In 1985, psychophysiologist Stephen LaBerge showed that our experience of time in a lucid dream is the same as when we're awake[38]. He did this by having dreamers count for ten seconds while dreaming, signalling at the beginning and end. This was confirmed in 2004, when it was also shown that muscle movements took longer to perform during lucid dreams[39].

2.2 Why do we dream?

While it's still not known why we dream, there are a number of theories. In 1983, biologists Francis Crick and Graeme Mitchison proposed the reverse learning theory, which states that dreams are needed for the brain to remove unnecessary information that it's received during the day[40].

There's also evidence of the opposite idea, that the brain needs dreams to remember information[41]. Studies have shown that you can improve your chances of recalling specific things if a memory of them if triggered while you're asleep[42]. Psychiatrist Jie Zhang suggested that dreams provide something analogous to a temporary storage space, which we can use to transfer short-term memories into long-term memories[43]. This is known as the continual-activation theory.

Another idea is that dreams have the purpose of providing us with simulations, which we can learn from. Neuroscientists Katja Valli and Antti Revonsuo argue that dreams evolved to prepare creatures for threatening events[44]. This is known as the threat simulation theory. Others argue that dreams provide more general simulations, which can also help with problem solving[45], and emotional control[46].

Research has already shown that people can learn new information in their sleep. In a 2012 study, volunteers were taught to associate different scents with different sounds while they were asleep, where the different scents led them to involuntarily produce different breathing patterns[47]. While the volunteers had no memory of this when waking, when they were played the different sounds while awake, their breathing pattern changed to match the smell they were taught to associate with it.

It's also possible that dreams are related to the defence mechanism of animals that pretend to be dead in order to escape threats, since both states are related to temporary paralysis[48].

Finally, dreams may not have an intrinsic purpose at all and may occur as the by-product of something else that is happening to our brains when we sleep. In the 1970s, psychiatrists J. Allan Hobson and Robert McCarley proposed the activation-synthesis theory[49]. This suggests that dreams are a by-product of REM sleep, caused by random signals from the brain stem being interpreted by the forebrain. The random nature of the signals causes dreams to appear somewhat nonsensical.

3. References

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