Vigilance stages [wakefulness, rapid eye movement (REM) sleep and non-REM sleep] correspond to clearly differentiated, discrete and global states of the brain. Their course is punctuated by brief activations, called arousals, which last from 3 to 15 s and belong to sleep structure. Either spontaneous or secondary to an identified stimulation, they reflect the sleeping brain ability to remain connected with his environment while preserving sleep continuity [1Halász P., Terzano M., Parrino L., Bódizs R. The nature of arousal in sleep J Sleep Res 2004 ;  13 : 1-23

Cliccare qui per andare alla sezione Riferimenti]. In scalp EEG recordings, they take the form of mixed patterns with various preponderance of fast or slow rhythms. Intracerebral recordings in drug-resistant epileptic patients provide unique insights in these transitional states as they allow to study local phenomena with a high spatial and temporal resolution. Using deep recordings in a wide sample of brain areas, we have previously shown that thalamic activity during arousal was very reproducible, whereas cortical arousals exhibited a high heterogeneity, according to the sleep stage and the cortical area considered, but also to the stimulation responsible for the arousal and homeostatic factors. Moreover, we observed that strictly local arousals could occur [2Peter-Derex L., Magnin M., Bastuji H. Heterogeneity of arousals in human sleep: a stereo-electroencephalographic study Neuroimage 2015 ;  123 : 229-244 [cross-ref]

Cliccare qui per andare alla sezione Riferimenti]. This heterogeneity could be responsible for the variability of perception and behavioral manifestations associated with arousals. As arousal remembering is usually poor, we studied if hippocampus activity during arousals and short awakenings (15 to 2min) could explain such low recall ability. We observed that hippocampal activity was different during NREM and REM arousals than during baseline sleep, and that this activation was modulated by the arousal duration. Such a temporal dynamic within hippocampal arousals could account for the progressive restoration of memory after awakening [3Koulack D., Goodenough D.R. Dream recall and dream recall failure: an arousal-retrieval model Psychol Bull 1976 ;  83 : 975-984 [cross-ref]

Cliccare qui per andare alla sezione Riferimenti]. It could be related not only to arousal remembering, but also to previous sleep-related mental activity (dreams) memorization, a critical duration of awakening being needed to enable full recall.