**6. Conclusions**

It is undeniable that highly specific sleep EEG rhythms and patterns of brain activation *actively* serve the memory, cognitive and psychological functions of sleep. The corresponding mechanisms involve brain plasticity at system, neural, synaptic, and genetic levels, and are closely related to the neurobiology of sleep. However, these mechanisms are *distinctly different* for certain memory, cognitive and psychological categories that sleep promotes, being *dissimilarly associated* with distinct sleep portions and sleep stages. Thus, both declarative and procedural memory consolidation and reconsolidation occur in earliest part of non-REM sleep/SWS by mechanisms of brain plasticity at system and neural levels, engaged in hippocampus-cortical relationships. Those, occurring during REM sleep appear more complex. Their pattern of brain activation engages a large set of areas, and possibly involves brain plasticity mechanisms at synaptic and genetic levels. The emotional 'fingerprint" of memory consolidation seems to be presented by connections between amygdala and cortical areas.

Further, the mechanisms involved in other cognitive functions of sleep appear different from those involved specifically in its memorizing effects. Also, it seems that sleep contributes to cognitive processes in a state dependent rather than in a trait dependent manner, but a complex interaction between both also can be suggested. Thus, since some of the EEG data imply a trait dependent role for sleep in cognitive abilities, most of the neuroimaging data indicate a state dependent role. However, these conclusions need further experimental evidence.

Importantly, different types of sleep mentality incorporated in different forms of dreaming production suggest a role for dreams in memory and cognitive processes. Data from such studies may open new perspectives of research and may provide new views about cognitive functions of sleep.

Finally, it can be concluded that the mechanisms underpinning memory, cognitive and psychological functions of sleep substantially contribute to human intelligence. Further investigation of the relationship between sleep and intelligence is clearly warranted.
