Figure 2. Sleep and attention are suppression states.

A) In the nematode, C. elegans, sensory neurons display less spontaneous activity and reduced responses to external stimuli during sleep-like states, and B) the timing of neural responses within a circuit becomes less coordinated [26, 28]; during wake, stimulation of a sensory neuron (black circle) leads to synchronous firing of two different interneurons (grey circles), resulting in a motor response, whereas the output is suppressed during sleep-like behaviour due to a time-lag in one of the interneurons [26]. C) Sleep in the fruit fly Drosophila melanogaster is characterized by an overall suppression of neural activity recorded in the local field potential oscillations in the brain [56, 57]. D) In Drosophila, attention towards a visual stimulus coincides with an increase in 20-50 Hz oscillations in the brain, whereas the same oscillations are suppressed when the stimulus is ignored [36, 37]. E) In humans, wake is characterized by high frequency activity, whereas during slow-wave sleep, large amplitude slow-wave oscillations indicate prolonged up-states (neural activity) and down-states (suppression of neural activity). F) Alpha oscillations (8-13) Hz are thought to provide a gating mechanism for visual attention in humans; brain regions with low amplitude alpha oscillations correspond with attention to a stimulus, whereas high amplitude or phase shifted alpha oscillations suppress the processing of unattended stimuli [40, 42-45].