Do I dream or wake up?

New insights into the sleeping brain

Functional magnetic resonance imaging of a healthy sleeping subject: During the tonic REM sleep, characterized by the brief absence of rapid eye movements, acoustic stimuli lead to a low activation of the corresponding sensory area in the brain, the auditory cortex (left picture). In contrast, in phasic REM periods, with eye movements, the brain is largely not stimulated by external stimuli, but shows a self-generated activity in far-reaching brain regions - an indication of the dream process (right picture). © MPI of Psychiatry
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As we sleep, our senses are largely cut off from the outside world - unless the alarm clock rouses us out of our dreams. Occasionally, however, it happens that the alarm ringing does not lead to awakening, but is incorporated into the dream life - and we oversleep. Why this is so, scientists have now revealed thanks to new measurement methods.

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So far, sleep phenomena have been studied mainly by measurements of brain waves. Scientists at the Max Planck Institute of Psychiatry in Munich have now for the first time measured these brain waves and at the same time demonstrated the activity of the sleeping brain with the help of the magnetic resonance imaging (MRI) imaging technique.

In the experiment, the scientists examined how the brain responds to acoustic stimuli such as a regularly recurring sound or piano music while sleeping. The subjects were exposed to noise of more than 90 decibels. The scientists also examined the typical early-morning REM sleep, which is particularly associated with intense dream life. This special state of consciousness, which owes its name to the occasional rapid eye movements, is characterized by high brain activity. In this sleep stage, we are subject to a temporary paralysis, not least to physically not dream possible experiences.

The exciting discovery: REM sleep can be divided into two distinct activity phases. When particularly many rapid eye movements were recorded, the activity was particularly high in different brain regions. Also, the areas that determine the emotional life, showed a high activity. However, the externally recorded sounds were obviously masked out by the brain in these phases. The researchers interpret the high brain activity as a neurological correlate of the - often intense - dream life. display

"Since in the meantime hardly any reaction can be made to external stimuli and the sleeper is almost defenseless, these intense phases appear in recurring, but usually very short periods, " explains Michael Czisch. Not least for the protection of the sleeping organism lie between REM sleep phases, in which the responsiveness to external stimuli is increased again - in these phases, the ringing of an alarm clock can reach us again.

(MPG, 26.03.2007 - NPO)