2.16 Dreaming and sleeping

Zebra finches replay the songs they’d learned as youngsters, during their sleep.This remarkable discovery was relayed to a science group discussion by Sally who had heard a talk at the UK Royal Society about animals dreaming. Scientists in Buenos Aires have found that finches’ brains show the same signals during sleep as they do when singing and their vocal chords move silently at the same time.  In effect, they appear to be dreaming.

Figure 1 Zebra finches. Image Credit: Michael Lawton, on flickr

An even  more explicit indication of animal dreaming was observed by a Brazilian team studying octopuses. Well-known for changing colour to suit their surroundings, they were found to also alter their colouring during active phases of their sleep, from a dull grey to bright orange. Various muscles twitched at the same time.

In another study, chimps who’d been taught the symbol for coffee in sign language were observed making the sign even while asleep. Could they be dreaming about coffee? Although it can’t be definitively proven that these cases are of dreaming (how could this ever be achieved?), the similarity with sleep patterns of us humans is certainly suggestive.

Figure 2 The Dream by Pierre-Cécile Puvis de Chavannes, 1883

These fascinating examples of research into animal psychology throw up a host of interesting questions; the discussion group wasn’t slow to pose them. If animals dream, do they experience emotions? What are emotions anyway? What are dreams – why do we have them? What makes us go to sleep in the first place? Do all animals sleep? This blog explores some of these interconnected issues.


Emotion is a contested area of science, with competing theories to explain it. Psychologists generally distinguish between primary (or ‘basic’) and secondary (or ‘complex’) emotions. There is no absolute consensus about this distinction or about which emotions are considered primary. Fear, anger, disgust, happiness, sadness, and surprise is one commonly cited set. These are observed in other primates and are generally considered to have evolved as our early ancestors adapted to their environment. These kinds of emotion are often associated with specific physiological changes such as increased blood flow to appropriate muscles, increased breathing rate and changes in facial expression. These can  switch on so rapidly they may activate our bodies before we are consciously aware of them. Behavioural changes such as smiling, running away or frowning often follow .

These physical responses may have evolved from actions that physically aided survival for our early ancestors. Widening the eyes to expand the field of view when facing  a fearsome threat or screwing up the nose and mouth in disgust to restrict facial openings when faced with foul air, for example. It may well have been a later development that these visible expressions became a means of communicating socially about emotions, such as the presence or absence of threat.

These conclusions about the physiological and behavioural aspects of emotion were arrived at through studies in which people and animals were visually observed and physiologically monitored. Our understanding has been significantly enhanced in recent years by the advance of neuroscience.  Sites in the brain have been identified that are associated with particular emotional responses.

One specific region, known as the limbic system, is associated with emotional learning and memory. Stimuli from the senses – through the eyes, ears, nose and skin –  are transmitted to the limbic system where they become associated with memories of previous emotional episodes. The system then exerts an influence on the body as a whole by sending out chemical signals through the nervous and hormonal systems as well as communicating with other parts of the brain. 

At this point in the discussion group, Sally saw an interesting possibility for deciding about the emotional life of other species. “Do other animals have a limbic systems?” she asked.  “If so, wouldn’t that mean they experienced emotions?”. “They must do” was Helen’s immediate response.

“My cats often express complex emotions, like wanting affection”. “But what if they don’t get it” Patrick questioned;  “do you think they feel disappointed?”. “No” responded cat-loving Jean, quick as flash; “they just walk all over your computer, instead”.

Figure 3 Kitten demanding attention. Image credit dougwoods via flickr

Studies on a wide range of species confirm Helen’s conviction. The limbic system is present in very many species; in fact it must be ancient in evolutionary terms as it is found across all  reptiles, mammals and even birds (which evolved from dinosaurs). One  theory suggests that it may have developed as a system for managing the neural circuitry required for the ‘fight or flight’ response. It therefore seems likely that many kinds of animal do indeed dream and may well experience emotion, at least of the basic or primary kind, since they have the brain structure for processing it, and have been observed, as in the examples above, behaving as they do while asleep.

The more complex secondary emotions we experience as humans vary more widely across cultures and between individuals. Some theories suggest they may involve different combinations of the various primary emotions. Grief, regret, and jealousy are examples. Feelings of grief, for example, may not express sadness alone, but also include elements of surprise and anger at a sudden loss.

“Is there a distinction between the ideas of emotion and feeling?” asked Patrick at this point. There is, at least in the lexicon of psychologists.  Emotion is understood to refer to bodily sensations, often triggered off unconsciously; whereas feelings are influenced by our thoughts and memories and are thus highly subjective. Emotion and feeling remain highly active areas of psychological and physiological research with many alternative theories co-existing to explain their origin and meaning.

It was to the issue of dreaming that discussion returned at this point. Why do we (or any other species) dream at all?


This area of science is also not settled; alternative theories are supported by evidence from different studies. What is agreed is that signals from the external world, through your senses are blocked from entering the dreaming brain and signals outwards from the brain instructing muscles to move are similarly inhibited. Despite this, your brain is highly active during the phase of sleep most strongly associated with dreaming, known as REM sleep. As dreamers, we know from experience just how lively our sleeping brains can be; sleep scientists measure this activity to study what’s happening physiologically.

One theory suggests that internally generated signals are being processed by your dreaming brain which treats them just as it would for external signals from the senses  when awake. Your brain does its best to make meaning out of these internal signals, as it would during wakefulness. The weirdness might then arise from the mixing of inputs from multiple brain areas, each of which may reflect the physical state of different parts of the paralysed body. Another theory suggests that the function of dreams is to reorganise or de-clutter the brain after the excess of information it has had to process during waking hours.

Theories, and the studies that underlie them, offer important clues about how the different phases of sleep are associated with the activity of nerve cells and the various chemicals that connect them – serotonin, histamine and dopamine, for example. Those who‘d love to know more precisely why we dream about particular people from our past or find ourselves in bizarre and unprecedented situations must remain frustrated for now. Such detail remains largely to be discovered; though some research suggests that imagery may reflect bodily conditions at the time. For example the feeling of being trapped may arise because sensory inputs are blocked and dreams about falling may be linked to activity in the brain’s balancing mechanism.

Although research is not yet be able to explain fully the extraordinary imagery with which our  dreams are filled, it is actively exploring the broader question of why we sleep at all. Understanding more about the disorders that impair sleep for many people is of major importance, for society in general as well as individuals. Members of the discussion group had ample anecdotal experience of this and were keen to discuss what they had picked up about ‘sleep hygiene’.


Sarah had been on a course about sleep therapy to help improve the quality of her sleep.  She had learned there is no simple rule about the  number of hours of sleep needed: it depends on the individual. Jean, however, finds she “can’t function when she sleeps for a few hours only”. Helen felt “an eight-hour rule for everyone doesn’t ring true: as an average it shouldn’t be interpreted as applying to every individual”. Amy wondered whether you should “go on the number of hours or on how tired you feel”. Discussion moved on to other kinds of difference between individuals. “For menopausal women it’s about temperature” commented Jean: “you just feel too hot to sleep”. “There are different sleep types” added Sarah: “the night owl and early bird. Society needs to accommodate these differences”. “Is it to do with shift-work and lighting” queried Jean. “Presumably early humans just slept when it was dark” she speculated.

Various methods are used to research sleep. In laboratory experiments, brain activity and other physiological factors are monitored and linked to behaviours such as moments of waking and movement of eyes and limbs. Epidemiological studies report on where and when sleep problems occur across a population; surveys analyse what people say about their sleep.

A review of research about sleep in the journal Pharmacy and Therapeutics summarises the key points arising from a wide range of studies. It makes clear that sleep is essential for healthy living in very many ways: for thinking clearly, being alert and attentive, consolidating memories and regulating emotion. The ideal length of time spent sleeping is somewhere between seven and seven and a half hours for the average healthy adult; but duration is not the only factor: timing of sleep may be just as important. Sleep quality is better when it’s aligned with our circadian rhythm (cycle of night and day) rather than upset by nightshift work, jet lag or other irregularities. As we know from personal experience, loss of sleep on a single occasion can soon be  made up, but, if repeated, the penalties of losing sleep can accumulate. Individuals vary in how harmful the effects of this are.

These established facts about sleep hygiene help guide our daily habits, and there are many  organisations and websites to support us in doing that. What does science have to tell us about the most fundamental question: how do our bodies manage to make us fall asleep? In other words: what is the physiology of sleep?

It’s a complex process, but fortunately the American Diabetes Association has recently reviewed and summarised the evidence about it. Simply staying awake during the day may seems a rather unremarkable fact of life, but it is, of course no accident. It’s the result of at least six different chemicals acting continuously to keep the outer parts of the brain (cortex) aroused. Serotonin, dopamine and noradrenalin are among the more familiar of these; all six are needed to avoid sleepiness. Given this active and sustained effort to keep us alert during the day, you’d expect that to fall asleep the chemicals that arouse would need to be blocked. You’d be right: that’s exactly what happens: neurons (nerve cells) in the various pathways that deliver these wakefulness chemicals are induced to inhibit their flow. Figure 4 indicates the pathways of chemicals that keep the brain aroused (orange) and those that inhibit this (blue).

Figure 4   Brain networks regulating sleep and wakefulness.
A the arousal system; B pathways that inhibit the arousal system

Adapted and Reprinted with permission from David W. Carley;Sarah S. Farabi, Physiology of Sleep; Diabetes Spectr 2016;29(1):5–9, https://doi.org/10.2337/diaspect.29.1.5. Copyright 2016 by the American Diabetes Association.

These blocking neurons get switched on by a cunning mechanism which measures how long you’ve been awake. A chemical called adenosine accumulates slowly throughout the course of the day in a particular part of the brain. When sufficiently abundant, this chemical (adenosine) interacts with receptors in the relevant neurons, switching them on and thus blocking the wakefulness chemicals. Interestingly adenosine is itself produced as energy is used up. It is a by-product of the breakdown of the universal energy molecule “adenosine triphosphate” (ATP) which releases energy into our bodies as it breaks up. Thus, the build-up of ‘sleep molecules’ results from the breaking up of ‘energy molecules’; a neat mechanism ensuring that the faster we consume energy the greater the impulse to sleep. It should be added that some other processes also influence wakefulness, most notably the body’s circadian clock which tracks the 24-hour cycle. This helps induce us to sleep not only when tired but also when the light fades.

We know from common experience that we don’t usually sleep in one continuous, uniform slumber from beginning to end. At some points we seem closer to wakefulness, at others, deeply immersed. Sleep researchers have investigated these phases and found two distinct kinds, known as REM and non-REM sleep. During the former, a chemical which stimulates anxiety (known as noradrenaline) in one part of the brain disappears, leaving us more or less free of anxiety. At the same time  however, other parts of the brain processing emotion and memory remain active. This combination of emotional activity and a stress-free environment appears to help our brains work through unhappy or stressful memories in a relatively peaceful way. REM sleep is associated with deeper immersive dreaming. In the course of a night’s sleep our bodies cycle through alternating phases of REM and non-REM sleep.

The chart in figure 5 shows the duration of the various phases over a 90-minute cycle. The depth of sleep is represented vertically.

Figure 5 The stages of one cycle of sleep. Image credit: Schlafgut via wikimedia

W is the wakefulness phase, R is REM sleep and N1, N2 and N3 , three stages of ever deepening non-REM sleep. As the horizontal time axis shows, some stages are very brief, others longer. In the course of a night the cycle may repeat four or five times. During the non-REM phases of sleep our heartbeat, breathing and muscle activity slow down. The deep sleep part of this phase (N3 in figure 3) is particularly important for the body and brain. Growth hormone is released during it, which helps repair tissues and bones. Deep sleep also helps regulate many important bodily processes, including functioning of  the immune system, consolidation of memory and metabolism of the sugar glucose.


A discussion that began with the simple question of whether or not animals dream has led into an exploration of both dreaming and sleeping. Both are active areas of research with many investigators looking into different aspects. Differing theories emerge from these studies, so the science is not settled. Nevertheless, the insights outlined above offer helpful insights to aid our understanding. The process that makes us fall asleep involves many different chemical pathways, mainly within the brain. We are dependent on all of them working optimally if we are to sleep easily. The rhythm of day and night also plays an important role, so disruptions to this can have negative consequences on the quality of our sleep. The overall message is that sleeping well is beneficial for many aspects of our wellbeing, both mental and physical. Ongoing research is helping clinicians devise treatments for the many disorders that can so easily occur.

© Andrew Morris 6st October 2022