The Biology of Dreaming

Dreams are memories of our semi-conscious thoughts and experiences that occur when we are asleep. As far as we know all mammals dream and some animals (but not Humans) can die from lack of it1. Us Humans spend on average, five to ten hours asleep per day, and one twelfth of our lives dreaming, whilst reptiles and other lower organisms do not dream2. Dreams occur because our brain is very sensitive to input and stimulation, and even as we sleep our neurones are firing. Our brain continues to interpret these 'random' inputs, often taking into consideration things that have been occurring recently in the persons' life, or things that the person, consciously or unconsciously, has brought to the forefront of their mind.

1. Why Do We Need to Sleep?

Many people feel that, despite 50 years of research, all we can conclude about the function of sleep is that it overcomes sleepiness.

Horne, J 3

People have put forward different reasons as to why we need to sleep. "Psychological theories are usually of a psychodynamic type, such as Jung's and Freud's, or of an information processing type, such as Calvin Hall's cognitive theory or Evan's computer theory"4. Psychodynamic theories are those that place most importance on the processing of unconscious thoughts, and more biological-mechanical theories place more importance on the ordering of all thoughts and memory.

Sleep deprivation experiments have found that we suffer a wide variety of problems, including hallucinations, if we force ourselves to stay awake. Luckily this is difficult to do however one student volunteer inexplicably managed to stay awake for 11 days (264 hours), amidst increasing attention from researchers. He experienced mild delusions from day 4 and tremors from day 7 but no psychosis. He done this with no drugs or coffee. His brainwaves changed in form and no longer displayed the alpha waves of wakefulness but he was still alert enough on day 11 to play games with the staff (and sometimes won). He suffered no long-term side-effects.5

2. The Processes of Sleep

Brain Waves:

On an ongoing basis - even during sleep - electrical signals are constantly flashing over the brain; these signals can be detected and measured by an encephalograph. [...] These devices show that electrical signals in the brain don't come steadily, but are produced in short bursts like a series of waves; the shape of the waves change with the activity level of the brain.

Brain waves are measured in up to 30 cycles per second. [...] During wakefulness, the waves are fast and small (called Alpha Waves); intense thoughts or walking around will produce faster, sharper, more-jagged rhythms characteristic of beta waves. In deep sleep, the brain produces large, slow delta waves, and theta waves are seen in babies and during sleep.

"The Brain Encyclopedia" by Carol Turkington (1996)

REM Sleep
Whilst falling into deep sleep, our brain goes through a series of stages. These stages cycle throughout the night, differing according to our different sleeping patterns and internal states. Different patterns of waves, as measured on ECGs, characterize different stages of sleep.

About 80 minutes after you fall into slumber, your activity cycle will increase slightly. The delta waves will disappear, to be replaced by the beta waves that signal an active or "awake" brain. Your eyes will begin to dart around under your closed eyelids as if you were looking at something occurring in front of you. This period of Rapid Eye Movements is called REM sleep. During the first sleep cycle, REM sleep usually lasts from 8 to 15 minutes. In subsequent cycles, REM sleep may last for 40 minutes or more. Overall, you spend from 15 to 35 percent of the night in REM sleep.

"Understanding Human Behavior" by James V. McConnel (1986)

Our eyes go through bursts of movement during REM sleep, sometimes with a few minutes in between bursts. During a full undisturbed night's sleep four or five periods of REM sleep is normal, each one being longer than the one before.

Sexual Arousal
During REM sleep, "most males experience an erection of the penis, and most females experience vaginal swelling and sometimes a hardening of their nipples. This sexual arousal typically occurs at the onset of each Stage 1-REM sleep period"[McConnel].

Muscle Inhibition
During REM our muscles are inhibited from moving although our brains still send impulses to them, but during other phases of sleep we're not disabled. Sleepwalking, therefore, occurs during Stage 4 deep sleep, a time when we have none of the beta waves that indicate reactive brain activity.

3. The Biology of Dreaming

"Dreams, in one form or another, tend to occur throughout the sleep period. [...] Generally speaking, most organized and detailed dreams occur during REM sleep"[McConnel].

The cortex is the most recent part of our brains to evolve, and is generally said to be responsible for our 'higher functions'. The interaction between our limbic system and the cortex is responsible for our conscious experiences of emotions and sensations. The cortex is 'highly active' during REM sleep, despite the fact that most external stimulation is cut off. According to the activation-synthesis model dreams are a result of our cortex/mind interpreting the 'random' and unregulated processes of sleep as if they were real events:

Book CoverNot only is the cortex isolated (unable to control muscles) but there is also inhibition of incoming signals produced by the sensory systems (so perceptions of the 'real' world are selectively attenuated) (input blockade). Hindbrain and midbrain structures, normally involved in relaying sensory information to the cortex, spontaneously generate signals (PGO waves) responsible for cortical activation and are also indistinguishable from signals which would normally have been relayed from the eyes/ears. [...] So the brain is very active during REM and dreams are a conscious interpretation (synthesis) of all this activity. The cognitive system, which organizes sensory information into the simplest meaningful interpretation when we are awake, processes all the internally generated signals as if they came from the outside world. In combination with oculomotor activity, PGO waves are sent to the visual and association cortex and the thalamus. [...].

"Psychology: The Science of Mind and Behaviour" by Richard Gross (1996)6

The activation-synthesis model of dream interpretation holds that the basically random source of electronic information causes the activation of specific areas of the cortex. This randomness underpins the resulting synthesis that our cortex tries to make of it.

Allan Hobson and Robert McCarley of Harvard University propose an "activation-synthesis" hypothesis, which explicitly rejects the Freudian, psychological interpretations. Instead, dreams, or at least some of their bizarre features, are seen as the associations and memories of the cerebral cortex that are elicited by the random discharges of the pons during REM sleep. Thus the pontine neurons, via the thalamus, activate various areas of the cerebral cortex, elicit well-known images or emotions, and the cortex then tries to synthesize the disparate images into a sensible whole. Not surprisingly, the "synthesized" dream product may be quite bizarre and even nonsensical because it is triggered by the semirandom activity of the pons.

"Neuroscience" by Bear, Connors and Paradiso (1996)7

Lucid Dreaming

According to Stanford psychologist Stephen P. La Berge, lucid dreaming is most likely to occur during the last dream cycles of the night. La Berge states that, during a lucid dream, you are aware that your "experiences" are dreams rather than reality, and you can remember the dream quite well after you have wakened. Sometimes you can even evaluate what is happening during the dream, and take an active role in resolving the conflict that occurs.

"Understanding Human Behavior" by James V. McConnel (1986)8

"Delusional Fear": A partially lucid dream from 2004 is an example: I realized I was 'in' a dream, and with that realisation I was able to modify what was happening in the dream (and break the laws of physics).

4. Infants' Dreams

An unborn baby at 32-36 weeks will have the thalamo-cortical neural networks in place that facilitate conscious awareness. Prof. Koch notes that an unborn baby at that point is almost permanently asleep and sometimes in REM sleep; although we do not know to what extent late unborn babies dream, there is evidence that they feel "the way we do when we are in a deep, dreamless sleep". Later:

Dream content is informed by recent and more remote memories. Longitudinal studies of dreaming in children by retired American psychologist David Foulkes suggest that dreaming is a gradual cognitive development that is tightly linked to the capacity to imagine things visually and to visuospatial skills. Thus, preschoolers' dreams are often static and plain, with no characters that move or act, hardly any feelings and no memories.

Prof. Christof Koch (2009) in Scientific American Mind (2009)9

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By Vexen Crabtree 2005 Mar 20
(Last Modified: 2009 Dec 08)
Parent page: The Biology of Dreams and Nightmares: And Vexen's Dream Diary

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References: (What's this?)

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Bear, Connors and Paradiso
(1996) Neuroscience. Published by Williams & Wilkins, Baltimore, Maryland, USA. The Amazon link is to a newer version. Mark F. Bear Ph.D. and Barry W Connors Ph.D. are both Professors of Neuroscience at Brown University, Rhode Island, USA, and Michael A. Paradiso Ph.D., associate professor.

Gooch, Stan
(2007) The Origins of Psychic Phenomena: Poltergeists, Incubi, Succubi, and the Unconscious Mind. My references are to the original edition published as "Creatures from Inner Space" (1984, hardback) by Rider & Company, London, UK. The edition linked to here is published by Inner Traditions 2007; information retrieved from Amazon UK on 2007 Dec 14. [Book Review]

Gregory, Richard L.
(1987) The Oxford Companion to the Mind. Oxford University Press, Oxford, UK. Quotes from 1987 reprint.

Griffin, Joseph
(1997) The Origin of Dreams. Hardback.

Gross, Richard
(1996) Psychology: The Science of Mind and Behaviour. 3rd edition. Published by Hodder & Stoughton, London UK.

McConnel, James V.
(1986) Understanding Human Behavior. Hardback 5th edition. Originally published 1974. CBS College Publishing, Holt Rinehart and Winston, New York, USA.

Turkington, Carol
(1996) The Brain Encyclopedia. From 1999 paperback edition published by Checkmark Books, USA.


  1. Bear, Connors & Paradiso (1996) p465. Added to this page on 2007 Feb 26.^
  2. Gooch (2007) p217.^
  3. Bear et al (1996) p465. The quote is taken from James Horne's book entitled Why We Sleep. Added to this page on 2007 Feb 26.^
  4. Griffin (1997) p1.^
  5. Bear, Connors & Paradiso (1996) p470. Added to this page on 2011 May 20.^
  6. Gross (1996) p86. Added to this page on 2006 Oct 08.^
  7. Bear, Connors & Paradiso (1996) p470-471.^
  8. McConnel (1986) p66.^
  9. Scientific American Mind (2009 Sep/Oct) article by Prof. Christof Koch "When Does Consciousness Arise?" Added to this page on 2009 Dec 08.^

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