Sleep and consciousness

Abstract

Sleep offers a unique opportunity to relate changes in brain activity to changes in consciousness. Indeed, if it were not for sleep, when consciousness fades in and out on a regular basis, it might be hard to imagine that consciousness is not a given but depends somehow on the way our brain is functioning. At the same time as changes in consciousness occur, brain activity undergoes major changes through an orderly progression of sleep stages, which can be identified by recording the electroencephalogram (EEG), eye movements (EOG), and muscle tone (EMG). Within each sleep stage, there are frequent, short-lasting electrophysiological phenomena, such as slow oscillations and spindles representing moments at which brain activity undergoes important fluctuations. There are also orderly spatial changes in the activation of many brain regions, as indicated by imaging studies. Importantly, similar brain activities occur in animals, and this has spearheaded detailed studies of the underlying neural mechanisms. This chapter will first examine how sleep is traditionally subdivided into different stages that alternate in the course of the night. It will then review the dreaming events we experience across sleep. Next, it will consider the neural correlates of sleep and wakefulness - the brain centers that determine whether we are asleep or awake and the mechanisms giving rise to the electrophysiological activities across sleep. It will review functional imaging studies of human sleep including research of regional metabolism using positron emission tomography (PET), functional magnetic resonance imaging (fMRI) correlates of -spontaneous brain rhythms, as well as resting-state functional connectivity studies. It will review recent experiments combining transcranial magnetic stimulation (TMS) and EEG that allow perturbing directly cortical neurons and recording with millisecond resolution the response across the cortical mantle in sleep and wakefulness. It will then discuss recent intracranial studies in humans that have provided evidence for the local occurrence of sleep oscillations in both the sleeping and the waking brain. The demonstration of local sleep changes the traditional view of sleep as a monolithic, all-or-none behavioral state and suggests that mixed and dissociated states are not just found in pathological conditions. This chapter ends with some open questions: when and why do we lose consciousness in sleep? Is consciousness in sleep (dreaming) more akin to perception (bottom-up) or imagination (top-down)? And why is sleep consciousness largely disconnected from the external environment?