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Sleep and Dreams

Within a day, all animals and humans show recurring periods of immobility, usually in a characteristic posture and environment, and associated with an apparent loss of responsiveness to environmental input—they sleep. In contrast to a comatose state, some sensory discrimination is preserved and sleep is reversible when strong stimulation is applied.

Historically, the views that sleep is simply rest and that dreams convey a special meaning has prevailed in most cultures. Dream contents were assigned a mystical dimension—they were seen as messages sent by a god or as representing states unavailable to waking consciousness and foretelling future events, an attitude still popular. Contrary to these subjective interpretations, but instead based on measurable changes of brain activity and information processing mechanisms, research activities in the past decades have clarified different states within sleep, have unraveled many active processes within sleep, and have highlighted neurobiological processes underlying a multitude of sleep-related phenomena in animals and in humans. Until now, several functions of sleep have been proposed, ranging from energy conservation, thermoregulation, and detoxification to brain plasticity processes resulting in functional or structural changes. Current knowledge substantiates that neuronal assemblies are active, reactivated, or even modified during sleep.

This entry reviews the classic and more recently applied methods to study sleep and sleep-related phenomena. These approaches allow us to describe the substantial changes in the brain's capacities to process external information and brain activation patterns across the different stages of sleep and to investigate the neuronal basis of dream mentation.

Methods

The current understanding of cortical activity across the different stages of waking and sleep mainly derives from surface electroencephalographic (EEG) recordings reflecting cerebral synaptic activity. As a consequence, EEG criteria are generally used to differentiate the increasing depth of sleep on slowing of the EEG rhythms. The formerly held belief of sleep as a cessation of brain activity and annihilation of consciousness was finally overthrown in 1953 when Eugene Aserinsky and Nathaniel Kleitman first described an active brain state recurring in regular intervals within sleep. This state is accompanied by a loss of voluntary muscle control and by rapid eye movements. It is therefore called rapid eye movement (REM) sleep or paradoxical sleep, as it shares many features of neuronal behavior with wakefulness but still represents a sleeping state with high arousal thresholds.

Electrophysiological Sleep Recordings

REM and non-REM (NREM) sleep stages can be differentiated based on recorded EEG, eye movement (electrooculographic [EOG]), and muscle activity (electromyographic [EMG]) criteria. Waking and consciousness rely on activity of neurons in the formatio reticularis (reticular formation) of the brain stem, forming the ascending reticular activating system projecting to the thalamus and cortex. Waking EEG recordings are dominated by fast frequency activity in the beta (15–30 Hertz [Hz]) and gamma range (30–80 Hz). The electrophysiological signs of NREM sleep—synchronized low-frequency oscillations with high amplitudes in the EEG recordings—seem to confirm the early notion of NREM sleep as a state of rest with cessation of intense cortical activity.

In contrast to NREM sleep, REM sleep shares many signs with wakefulness such as fast-frequency and low-voltage cortical EEG, including gamma oscillations. Whereas slow-wave sleep depends on thalamocortical synchronization, specialized cells in the brain stem called REM sleep-on cells initiate and generate signs of REM sleep. From an evolutionary point of view, REM sleep is exclusively found in birds and mammals—that is, it is unique to endo-therm animals with well-developed brains that also express slow-wave activity, with the exception, for example, of dolphins. During ontogenesis (development of the individual), REM sleep is found in relatively high amounts during early development, and a link to brain maturation in phylo- and ontogenesis was proposed. Because of circadian modulation, REM sleep episodes are longest and REM density is strongest at the time of minimal body temperature, usually in the early morning hours. The concurrent inhibition of spinal motoneurons led to the depiction of REM sleep as a “highly activated brain in a paralyzed body,” also lacking proper input processing.

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