Are we what we dream

For most people the term ‘sleep’ conjures thoughts of rest, relaxation and recovery in preparation for the next day. However, even in the deepest phases of sleep, the brain is far from resting. Indeed, it is occupied with performing vital functions and activities that we have known about for a long time, but the purposes of which have remained elusive. Sleep is an evolutionarily conserved activity that occurs in a wide variety of animal species. The amount needed is proportional to brain development and lack of it can be fatal. Two types of sleep that alternate during sleep periods have been classified in mammals: slow-wave sleep (SWS), which is characterized by slowing of bodily functions, diminished responsiveness to sensory stimuli and EEG recordings that show large-amplitude and low-frequency oscillations; and rapid-eye-movement (REM) sleep, which is characterized by lack of muscle tone, sudden eye movements, irregular respiration, and low-amplitude and rapid oscillations on EEG recordings. REM sleep has been proposed to be required for the consolidation or maintenance of information acquired during wakeful periods. To examine this hypothesis, Maquet et al.1xExperience-dependent changes in cerebral activation during human REM sleep. Maquet, P. et al. Nat. Neurosci. 2000; 3: 831–836Crossref | PubMed | Scopus (396)See all References1 used brain-imaging techniques to determine if well-defined activities performed while awake, could change the regional pattern of brain activity during REM sleep.Using positron emission tomography (PET) and regional cerebral blood flow (rCBF) meas-urements as an indication of local synaptic activity, the cerebral activity of experimental human subjects was recorded under various conditions. Subjects were either trained in a serial reaction time (SRT) task or untrained, and rCBF measurements were taken at rest, during the training and during sleep after training. The subjects were shown to improve their reaction times up to a plateau during the day but, after sleep, they were able to improve reaction times beyond those of the previous day. The pattern of brain activation observed during testing was consistent with the use of motor, processing and visual cerebral activities associated with the task. Significantly, in REM sleep, a subset of the regions activated during SRT task performance were more active in trained rather than in non-trained subjects. These regions included the cuneus, the adjacent striate cortex (bilaterally), left premotor cortex and mesencephalon. This effect was not reproduced during SWS.These data indicate that re-activation of brain areas that are stimulated during wakefulness occurs during REM sleep. The re-activation of brain areas involved in processing the SRT task during REM sleep of trained subjects, and the subsequent improvement in task performance, supports the theory that some reprocessing, fine tuning or consolidation of acquired information and skills is taking place during REM sleep.However, without measurements from a population of subjects trained in the task and deprived of REM sleep, the contribution of REM sleep to the improved SRT task performance cannot be confirmed. Identi-fication of the molecular mechanisms involved in REM sleep, such as the recently discovered hypocretin signaling system, in conjunction with physiological studies, will enable a better understanding of the roles of this complex behavior. The formation and consolidation of memories defines us as individuals. Thus, REM sleep, along with other processes, might be an essential mechanism in the determination of who we are.