Jung & Haier (J&H) propose a theory based on an excellent review of modern neuroimaging data. In their model, the parieto-frontal (P-F) cortical areas are mainly implicated in human intelligence, although other brain regions also can be engaged. In particular, the amygdala is shown to be involved in emotional intelligence (Bar-On et al. Reference Bar-On, Tranel, Denburg and Bechara2003; Brierley et al. Reference Brierley, Medford, Shaw and David2004; Shaw et al. Reference Shaw, Lawrence, Radbourne, Bramham, Polkey and David2004). Combining strong neuroimaging evidence with developmental and neurogenetic implications, J&H address the fundamental question of how brain and behavior are associated through the expression of intelligence and reason in an advantageous way.

However, one very important issue is generally ignored in their theory, namely the role of sleep for human intelligence. In fact, we spend a substantial part of our life in sleep. A large body of data clearly demonstrates that sleep does have an essential role for different types of learning and memory, and, likewise, for our intelligence (Hobson Reference Hobson2005; Maquet Reference Maquet2001; Stickgold Reference Stickgold2005; Stickgold & Walker Reference Stickgold and Walker2005). Moreover, sleep is an active brain state consisting of different stages (Steriade & Hobson Reference Steriade and Hobson1976), and it is recently becoming evident that these different sleep stages affect various types of human cognition dissimilarly. For example, whereas rapid eye movement (REM) sleep has been implicated mostly in procedural learning and memory, non-REM (NREM) sleep has been shown to modulate the consolidation of declarative (or explicit) learning and memory (Born et al. Reference Born, Rasch and Gais2006; Plihal & Born Reference Plihal and Born1999; Walker & Stickgold Reference Walker and Stickgold2004). More specifically, the declarative memories benefit from stage two of NREM sleep, as well as from the sleep spindles in this stage and their grouping by slow cortical oscillations (Clemens et al. Reference Clemens, Fabo and Halasz2005; Gais et al. Reference Gais, Molle, Helms and Born2002; Marshall et al. Reference Marshall, Helgadottir, Molle and Born2006; Schabus et al. Reference Schabus, Gruber, Parapatics, Sauter, Klosch, Anderer, Klimesch, Saletu and Zeitlhofer2004). Some empirical results concerning the association between sleep and intelligence are to be further emphasized. For example, it has been elegantly demonstrated that sleep inspires insight, an important aspect of human intelligence (Wagner et al. Reference Wagner, Gais, Haider, Verleger and Born2004), and revives emotional representations (Wagner et al. Reference Wagner, Fischer and Born2002; Reference Wagner, Hallschmid, Rasch and Born2006). More importantly, recent studies show a strong and positive correlation between individual intelligence and the amount, quality, and quantity of sleep (Alchanatis et al. Reference Alchanatis, Zias, Deligiorgis, Amfilochiou, Dionellis and Orphanidou2005; Bodizs et al. Reference Bodizs, Bekesy, Szucs, Barsi and Halasz2002; Reference Bodizs, Kis, Lazar, Havran, Rigo, Clemens and Halasz2005; Schabus et al. Reference Schabus, Hodlmoser, Gruber, Sauter, Anderer, Klosch, Parapatics, Saletu, Klimesch and Zeitlhofer2006). Also, preceding learning has been found to produce quantitative electroencephalographic changes during subsequent sleep periods (Huber et al. Reference Huber, Ghilardi, Massimini and Tononi2004; Molle et al. Reference Molle, Marshall, Gais and Born2004). Therefore, there is unambiguous evidence that sleep strongly impacts on human intelligence.

Concerning further the nature of human intelligence, it is well documented that brain physiology and neuromodulation considerably differ across sleep-wake stages. Importantly, the majority of neurotransmitters involved in these regulatory processes originate from nuclei located in the brain stem, projecting their activity to the cortex; in this manner, they modulate cortical activation (Gottesmann Reference Gottesmann1999; Hobson et al. Reference Hobson, McCarley and Wyzinski1975; Pace-Schott & Hobson Reference Pace-Schott and Hobson2002). Given the relevant contribution from subcortical regions to cortical plasticity, it may be a limitation to regard the P-F cortical areas as the only neuroanatomical source of human intelligence, as J&H propose.

Further, the processes involved in sleep-wake regulation also have been implicated in the states of consciousness (Hennevin et al. Reference Hennevin, Huetz and Edeline2007; Hobson et al. Reference Hobson, Pace-Schott and Stickgold2000; Tononi Reference Tononi2005). Accordingly, the brain activation, the information flow, and the neurochemical mode of modulation, all in combination, may determine specific states of mentation (Hennevin et al. Reference Hennevin, Huetz and Edeline2007; Hobson et al. Reference Hobson, Pace-Schott and Stickgold2000). Therefore, the processes of sleep-wake regulation and their corresponding states of consciousness may have much in common with human intelligence.

Taking all these considerations together, it may be concluded that the neuroimaging data incorporated in the model of J&H could hardly explain the nature of all aspects of human intelligence. This limitation leads to the following question: How do sleep, states of consciousness, and their regulatory mechanisms relate to human intelligence? This question is certainly of relevance and mandates further investigations.