Conclusion

In this chapter we briefly reviewed the neurophysi-ology of normal human sleep, including the electro-physiological and neuroendocrine characteristics of the REM/NREM sleep stages, and described the elec-trophysiological assessment of sleep disorders with polysomnography (PSG). We also discussed the organization of sleep (sleep architecture) in young healthy adults, and reported the effects of age, psychiatric disorders, and psychoactive medications on sleep architecture. Finally, we presented several neuroim-aging techniques, including fMRI, PET, and hd-EEG, which have been increasingly utilized in sleep research to explore the neuroanatomy of normal sleep and the effects of sleep deprivation on cognitive functions.

The science of sleep has progressed tremendously in the last 50 years, from the early description of NREM/REM sleep stages, through the investigation of sleep architecture in healthy subjects as well as in neuropsychiatric patients with various sleep disorders, and now to the study of the neural correlates of sleep activity with state-of-the-art neuroimaging techniques. In particular, the exquisite spatial and temporal resolution provided by neuroimaging tools such as fMRI and hd-EEG has recently enabled us to explore sleep activity in greater detail and to begin to address some of the fundamental questions regarding the function of sleep.

For example, what is the exact relationship between sleep and fundamental waking activities, such as memory and learning? Are local changes in sleep activity (occurring in the same area where waking plastic changes have taken place) more relevant to the function of sleep than the overall global activity? Are specific brain areas more in need of sleep, and therefore more sensitive to the effects of sleep deprivation? Are some sleep-specific rhythms (e.g. sleep slow waves, sleep spindles) more involved in implementing these waking-related plastic changes in the healthy brain? And are deficits in some of these sleep EEG rhythms relevant to the neurobiology of psychiatric disorders?

In general, the study of spontaneous neural activity during sleep offers some important advantages for investigating brain function in psychiatric patients. Sleep recordings minimize possible confounding factors related to waking activities, including changes in the level of attention, decreased motivation, or cognitive capacity, and the presence of active symptoms. Intriguingly, in a recent study employing hd-EEG during sleep it was found that sleep spindles were markedly reduced in schizophrenic patients compared to healthy subjects as well as psychiatric controls, and that these spindle activity deficits provided a 90% separation between schizophrenics and subjects from the other two groups [44]. These findings, whose relevance will be established by future studies on larger populations of psychiatric patients and healthy subjects, show the importance of sleep research in psychiatry, and underscore how imaging techniques applied to the sleeping brain may contribute not only to diagnoses but also, ultimately, to a better understanding of the neurobiology of neuro-psychiatric disorders.

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