Monday, April 30, 2007
To Sleep: Perchance To Dream
Half a century ago, scientists knew very little about dreams, They could not even say what proportion of the night was devoted to dreaming. But since the 1950s, they have started to unlock the closed door that hides roughly a third of our lives. This has been made possible by the invention of the electroencephalograph in 1929, by the German psychiatrist Hans Berger. The name means 'electric head writing', and describes quite aptly how the secrets of sleep are revealed.
The EEG, as it is known for short, is wired to the heads of volunteer sleepers in a laboratory. Groups of brain cells produce small electrical charges in patterns, and these can be detected by electrodes placed on the scalp. The electrical charges are converted into vibrations that are powerful enough to activate a number of pens, on electronic arms, to trace the movements on paper.
Signs of sleep
EEGs have detected that we go through several well defined stages of sleep in 90-minute cycles. When we are active with our eyes open, the EEG does not pick up any discernible pattern. But as soon as we close our eyes and begin to 'drift off' into sleep, alpha waves start to vibrate at 8 to 12 times per second, showing up on the EEG as a series of spikes. The onset of sleep itself is shown by the tracings of theta waves at a slower 3 to 7 times per second. Alpha and theta waves are associated with deep relaxation, creativity and tranquillity. Yogis and other experienced practitioners of meditation can, with practice, produce this level of relaxation when awake.
The deepest stage of sleep begins as the brain slows down to delta waves, vibrating at only 1 to 2 pulses per second. But then a curious thing happens: the EEG pens start tracing irregular spikes - similar to those recorded during wakefulness - as facial muscles and other parts of the body start to twitch.
This sudden of activity after a period of deep calm led scientists to name this stage 'paradoxical sleep'. But the darting about of the eyeball behind the eyelid, which is characteristic for this phase of sleep, gave it its better known name of rapid eye movement (REM) sleep. Scientists can monitor this stage on the EEG, and have discovered that if a person is awakened during REM sleep, he or she will be able to remember a dream eight times out of ten. By comparison, if interrupted during non-REM sleep, only about one in ten people recalled having had a dream. So it is thought that rapid eye movements are the response of the dreamer to the visual events of his or her dream-world.
We experience REM sleep about five times a night. The cycle of non-REM followed by REM sleep normally lasts about 90 minutes, but as the night goes on, the proportion devoted to REM sleep increases. So while our first dream may take only 10 or 15 minutes, just before waking we may have been engaged in a 45-minute epic.
DID YOU KNOW? Scientists think that babies dream in the womb. Ultrascan tests to monitor foetal development during pregnancy have picked up rapid eye movements of the type associated with dreaming sleep.