Sleep is commonly regarded as a state of (reversible) ‘disconnection’ from the external environment, both on the sensory input and motor output sides. In this perspective, dreams are seen as ‘islands of consciousness’ generated and unfolded by the brain independently from any external influence. However, a growing body of evidence indicates that sleepers are not entirely disconnected from their environment. In fact, for instance, salient stimuli, such as those associated with a particular affective load, may trigger different responses in the sleeping brain or even lead to full-fledged awakenings relative to non-relevant stimuli. This simple observation indicates that even during sleep, the brain may remain able to process some features of incoming stimuli and classify them based on their potential relevance for the sleeper. Furthermore, the combination of sensory stimulation and serial awakening protocols revealed that dreamers may consciously perceive many external stimuli in their dreams. However, such stimuli are only rarely recognized as coming from the external environment, as they are typically integrated into -and become part of- the ongoing oneiric experience. 

Here I will discuss the results of studies conducted in our and other laboratories that support the view of sleep as a sentinel state in which monitoring and classification of external stimuli are continuously performed. Moreover, I will present evidence suggesting that dreaming, together with subcortical and cortical gating processes, may be one of the most important lines of defense between remaining asleep and being awakened.