Sleep
Questions Received:
Responses:
How do dreams relate to the nervous system?
20th April 1999
"To sleep: perchance to dream: ay, there’s the rub; For in that sleep of death what dreams may come..."
(Shakespeare: Hamlet, Act III, scene i)
Sleep is a periodic, recurring process during which brain activity changes. However, sleep is not a passive state - the brain remains active although in different ways compared with the waking state. When brain activity is monitored by way of electrodes attached to the scalp (electroencephalography), four levels of sleep can be distinguished. As sleep becomes deeper, the wave-like patterns of changing electrical activity become slower. Although interactions with the outside world become reduced during sleep, the isolation is not complete. Some sensory stimuli can still reach the cortical areas of the brain responsible for their processing and some motor activities can be initiated.
About an hour-and-a-half after falling asleep the tracings take on an appearance that is similar to those when we are awake, and the closed eyes begin to move rapidly from side to side. This is known as rapid eye movement sleep, or REM sleep. It is during REM sleep that we have vivid dreams. Brain temperature rises slightly, heart rate and blood pressure increase, respiration becomes more rapid, and gastrointestinal movements cease. These are indications of an increased activity in the sympathetic division of the autonomic nervous system. Apart from activity in the muscles controlling eye movements and muscles in the middle ear, contractions of skeletal muscles throughout the rest of the body are generally suppressed during REM sleep. However, there are occasional twitches of the limbs and face and interuptions in the pattern of breathing during REM sleep. After 5 to 15 minutes of REM sleep we revert to a deeper level of sleep again. This cycle is repeated another 4 or 5 times during the night. (It is interesting that day-dreaming has a similar cyclical pattern when we are awake in a relatively unchanging environment.)
REM sleep has been detected in all warm-blooded animals - mammals and birds. It is the dominant form of brain activity in fetuses before birth and in prematurely-born babies. The other patterns of brain activity associated with sleep and wakefulness gradually emerge to accompany the REM activity. After birth, and as we get older, we generally sleep less and spend less time dreaming. Although dreaming is often considered to be synonymous with REM sleep, there is convincing evidence of mental activities occurring during non-REM sleep too. In general, these activities are more conventional and less flamboyant than the dream-sequences experienced during REM sleep.
The possible function of dreaming continues to be discussed. The psychoanalyst Freud suggested that dreams are an expression of unfulfilled desires arising in our subconscious. Another view is that dreams play a part in the processing of recent experiences during which some memories are stabilised and others discarded. Dreams might also allow rehearsal of complex thought processes in advance of situations where they will be needed - this has been suggested as an explanation of how instinctive behaviours can be executed nearly perfectly the first time. Dreaming is vital to mental health, and if people are deprived of the opportunity to dream they often develop psychological symptoms such as hallucinations and paranoia. These changes are reversible when sufficient REM sleep takes place again.
Another important issue is the relationship between dreams and consciousness. Since the time of Descartes scientists have divided the world into material and mental realms and focused their attention and efforts predominantly on the material world. Consciousness has been considered largely inaccessible to study by scientific methods until comparatively recently. It is now referred to as ‘the hard problem’ - how is it that patterns of brain activity involving huge numbers of interconnected neurons can give rise to this quality we experience as consciousness? Dreaming appears to resemble the normal state of consciousness when we are awake, and yet has significant differences. Dreaming is now thought to be a particular level of consciousness.
Reading
For a useful review see:
Kelly, D. (1981) Physiology of sleep and dreaming. In: E.R. Kandel and J.H. Schwartz (editors) Principles of neuroscience. London: Edward Arnold (Publishers) Ltd. Pp. 473- 485.
Is it possible to be asleep with ones eyes open?
12th July 2001
The simple answer is yes, although generally our eyes close as we fall asleep in order to protect our corneas and do not open again until we re-awake. Open-eyed sleeping is called lagophthalmos and can be linked with a number of different causes and situations. For example, it can be associated with alcohol intoxication, the use of hypnotic drugs, damage to the branch of the facial nerve which supplies the orbicularis oculi muscle, scarring of the eyelids, and metabolic disorders such as thyroid dysfunction (Lyons and McNab, 1990). There is some evidence that open-eyed sleeping can be a family characteristic. Most commonly in lagophthalmos, only one eye remains open during sleep. It may be relevant to note that birds and aquatic mammals have the ability to sleep with half their brain at a time, keeping the other half alert and the corresponding eye open (Rattenborg, Lima, and Amlaner, 1999). In the case of birds, this is probably a defence mechanism, allowing the bird to remain alert to danger, while in the case of seals and dolphins this skill allows the animal to surface for breathing while asleep.
Reference
Lyons, C.J., and McNab, A.A. (1990) Symptomatic nocturnal lagophthalmos. Australian and New Zealand Journal of Ophthalmology, 18(4), 393-396 (Nov).
Rattenborg, N.C., Lima, S.L., and Amlaner, C.J. (1999) Half-awake to the risk of predation. Nature, 397, 397-398 (Feb 4).