The hypothesis that dreaming is involved in off-line memory processing is difficult to test because major methodological issues have to be addressed, such as dream recall and the effect of remembered dreams on memory. It would be fruitful – in addition to studying the ancient art of memory (AAOM) in a scanner – to study the dreams of persons who use AAOM regularly.

Dream research shows sex differences in dream aggression that fit very well with the findings for waking-life aggressive behaviour. Dream studies are a valuable tool for investigating variables underlying the sex difference in aggression. One might argue that studying dream aggression might be even more promising because aggression in dreams is not socially labelled, as being aggressive in waking life is.

From the perspective of modern dream research, Freud's hypotheses regarding repression and dreaming are difficult to evaluate. Several studies indicate that it is possible to study these topics empirically, but it needs a lot more empirical evidence, at least in the area of dream research, before arriving at a unified theory of repression.

In this commentary the “incredibly robust” evidence for the relationship between sleep and procedural memory is questioned; inconsistencies in the existing data are pointed out. In addition, some suggestions about extending research are made, for example, studying REM sleep augmentation or memory consolidation in patients with sleep disorders. Last, the possibility of a relationship between dreaming and memory processes is discussed.

All five target articles are of high quality and very stimulating for the field. Several factors such as dream report length and NREM/REM differences, may be affected by the waking process (transition from sleep to wakefulness) and the recall process. It is helpful to distinguish between a model for REM sleep regulation and a physiological model for dreaming. A third model accounting for cognitive activity (thought-like dreaming) can also be of value. The postulated adaptive function of dreaming in avoidance learning does not seem very plausible because the two major basic assumptions (specificity of dream content and benefit of negative dreams) are not clearly supported by modern dream research: The critique of studies investigating memory consolidation in REM sleep is justified. Future studies integrating the knowledge of memory processes and sleep research will shed more light on the role of sleep, especially REM sleep in memory consolidation.

[Hobson et al.; Nielsen; Revonsuo; Solms; Vertes & Eastman]