From a subjective viewpoint, it can be difficult to believe that dreams have any deep meaning. One of my most memorable dreams involved the actor Tom Selleck, a sea monster and a talking duck. If this was supposed to be a message from my subconscious, its significance was certainly lost on me. Indeed, reasons for dreaming have puzzled philosophers and scientists for many years. We spend nearly twenty-five years of our lives asleep, and dreams are a central part of that experience. It stands to reason that they should serve a purpose. While the debate on the purpose of dreaming is far from over, at last scientists have intriguing insights into how the brain spins its nightly adventure tale.
Dreams and REM Sleep
About 90 minutes after the onset of sleep, several abrupt physiological changes occur. Electroenchepalogram (EEG) readings become desynchronized, showing a low-voltage, fast activity pattern similar to that of the wakefulness. Despite the near-waking levels of brain activity, a person in this stage of sleep is the most difficult to arouse. Dubbed "paradoxical sleep," this stage is characterized by desynchronized cortical activity, rhythmic hippocampal activity, and loss of core muscle tone. Only the skeletal muscles controlling movements of the eyes, the middle ear ossicles and respiration are not paralyzed. In 1957, William Dement and Nathaniel Kleitman described the association of rapid eye movements (REM) associated with this stage of sleep, and since then it has become known as REM sleep. These rapid eye movements appear to be driven by phasic bursts of electrical activity that can be recorded in animals from a variety of structures in the brain stem, the thalamus and the auditory cortex. These monophasic sharp waves originate in the pons and travel through the lateral geniculate nuclei to the occipital cortex. Called PGO (pontine-geniculate-occipital) spikes, they correlate perfectly with the eye movements recorded in REM sleep.
Dement and Kleitman were curious to see whether the rapid eye movements were associated with dreaming, so they did a controlled study comparing subjects' responses to awakening from REM sleep versus awakening from non REM sleep. They found that REM sleep was highly correlated with dreaming; people awakened from REM sleep reported dreaming 80% of the time, whereas people awakened from non-REM sleep reported dreams only 7% of the time.
