Sleep is a phenomenon of astounding complexity, which makes it difficult to understand and even define unequivocally. It can be viewed as behaviour, a brain state and a process, which are intricately interrelated, and manifest themselves at many distinct spatio-temporal scales. Sleep is regulated by circadian time, preceding sleep-wake history, and, while asleep, the brain switches periodically between two markedly different states – NREM sleep and REM sleep, which are distinguished by specific types of brain activity. Specifically, a closer look at NREM sleep reveals that it is characterised by a regular occurrence of local and global slow cortical oscillations, visible at the level of the EEG as slow waves. Throughout NREM sleep, especially during its lighter stages and towards a transition into REM sleep, another type of activity is apparent, so-called sleep spindles. These involve the thalamus and through dynamic cortico-thalamic interactions emerge quasi-independently at specific brain locations and never across the whole brain at once. In contrast, during REM sleep the brain is about as active as it is in waking, and the EEG in both humans and animals is dominated by theta- and faster rhythms, which arise from bidirectional interactions of cortical, hippocampal and subcortical networks. Are all these sleep-related phenomena related to each other and what is the functional meaning of the overall complexity of the sleep process?
Our aim is to understand the mechanisms governing the spatio-temporal dynamics of brain activity during sleep. This will help us to understand not only what sleep is, but also why it is necessary.