We sleep through one third of our life, and dreams dominate two of the eight hours that we typically sleep during a day—but why we do both is a continuing mystery. Sleep is basically a brain phenomenon, since other body organs and (brainless) plants don’t sleep. Its biological importance is underscored by the fact that death occurs more rapidly because of sleep deprivation than starvation.
The year 1953 marked the discovery of both the structure of DNA and rapid eye movement (or REM) sleep, the period during which most dreaming occurs. Our understanding and manipulation of genetics has since advanced tremendously, but our understanding of much of the sleeping and dreaming process in both humans and animals remains tentative. Freudian beliefs that dreams represent repressed emotions tended to bias early thought and research, but recent advances in neuroimaging technologies are opening up research possibilities into sleeping and dreaming that didn’t exist earlier.
J. Allan Hobson is a pioneer researcher on the physiology of sleeping and dreaming. 13 Dreams Freud Never Had: The New Mind Science (2005) is his most recent book. It’s an intriguing autobiography that will appeal mostly to folks who have followed his career and the history of sleep/dream research.
Hobson always recorded his dreams immediately upon awakening, since dreams quickly fade if we don’t immediately contemplate, record, or relate them. He then analyzed his dream journals and those of others in order to discover the relationship between our dreams and conscious experiences. This research led to many discoveries, such as that dreams are typically about events that occurred several days earlier rather than during the preceding day, and represent only short memory fragments and not entire experiences. Less than 4% of a dream’s content is an actual replay of a waking experience. Further, contrary to what most people think, erotic feelings occur only rarely in dreams.
Hobson selected and described thirteen different kinds of dreams that he had recorded over the years, and wove them into an autobiography and discussion of what he believes about sleeping and dreaming.
What Scientists Know about Sleeping and Dreaming
Current theories of the purpose of sleeping and dreaming generally incorporate the need to restore depleted molecules or damaged tissue, conserve energy, process information (such as memory), and avoid predators. It’s important to detour the traffic off a road that’s being repaired, and it’s similarly important to shut down interference from our sensorimotor systems while our brain carries out necessary maintenance tasks. Several brain structures and melatonin fluctuations regulate our biological clock’s approximately 24-hour circadian rhythm.
Melatonin is a hormone that’s synthesized in the pineal gland. Drowsiness and sleep occur when melatonin levels increase. It’s normally produced during the evening and night and used up during the day. Jet lag occurs when our normal day is lengthened or shortened by rapid flight across several time zones. An extended form of jet lag affects many early adolescents as they go through the hormonal adjustments of puberty. It’s like they’re psychologically living two time zones west of where they actually live—sleeping and waking later than they formerly did.
Four to six approximately 90-minute periods characterize our nightly sleeping-dreaming cycle. Each period includes a sequence of two alternating states: (1) a mid-to-deep sleep state characterized by slow synchronized delta wave activity, followed by (2) a more active brainstem-initiated REM sleep state in which much of our brain wave activity resembles an awake state. During this state, eyeballs move rapidly, body movement is inhibited (so we don’t act out our dreams), and male and female genitals are aroused.
We often awake from a frightening dream at about 3 AM. One could think of waking as a survival strategy when things look bleak—wake up and thus escape the threatening dream situation we’re confronting.
REM sleep is more prevalent in infants than adults, and so it may be developmentally important. Non-REM sleep is characterized by reduced activity in such regulatory functions as circulation and respiration, but increased activity in the production and release of hormones. For example, the release of growth and sex hormones occurs principally during sleep, and this may explain why adolescent sleep is typically so long and deep.
Glycogen, found in glial cells, is a molecule that especially interests sleep researchers. It’s the stored form of glucose, the brain’s energy source. Like gas in a car, glycogen stores are depleted by activity, and so must be replenished during non-active periods (such as sleep or naps) during which sensorimotor activity is inhibited. The complex chemical replenishing process requires periods of brain activity but not wakefulness, and this may help to explain the active nature of REM sleep.
Similarly, the neurotransmitters acetylcholine and dopamine seem to be selectively released during REM sleep, and norepinephrine and serotonin are selectively released while we’re awake but suppressed during REM sleep. Alcohol suppresses REM sleep.
Dreaming continues to be enigmatic, although earlier Freudian views of primal meanings are giving way to biological explanations. Most dreaming occurs during REM sleep, and scientists have noted increased activity during dreaming in brain areas that process vision and emotion, and decreased activity in areas that process smell and taste, and rational thought and attention (such as in the frontal lobes). This tends to concur with the emotional, visual, irrational, distorted content of dreams. Dreaming may thus give our normally rational brain an opportunity to imagine and rehearse solutions to life’s problems without being inhibited by factual information and rational thought. Further, this unconscious exploration may enhance subsequent conscious creativity.
Sleeping and dreaming may also play an ill-understood role in the consolidation, editing, and erasing of memories (and especially procedural or skill memories). Memory formation and editing involve synaptic alterations in the neural networks involved in processing a memory, and such alterations are more easily effected during periods in which a brain isn’t consciously active in thought and behavior.
Biological studies of sleeping and dreaming states have led to the development of medical interventions for various sleep disorders, and we can anticipate more success in this in the years ahead.
Time to take a nap.