Figure 2.

Overview over sleep stages. (A) Sleep, defined as a readily reversible state of reduced responsiveness to the environment, is a phenomenon reported in all animals—from humans to flies and mollusks—that have been systematically examined so far (Borbély and Achermann, 2000; Huber et al., 2004b; Cirelli and Tononi, 2008; Vorster et al., 2014). Human sleep is traditionally dichotomized into rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NonREM) sleep and prototypically consists of 90 min cycles, during which NonREM sleep and REM sleep alternate. REM sleep (green) is characterized by rapid eye movements, a mixed frequency EEG and an inhibition of muscle tone. NonREM sleep can be further subdivided into sleep stages 1–4 (S1–S4), which resemble the depth of sleep, inasmuch as responsiveness to external stimuli is minimal in sleep stage 4. S1 is a transitory stage that only makes up a small amount of a night's sleep (<10%). S2 is determined by the occurrence of sleep spindles (waxing and waning activity of 10–15 Hz) and K-complexes (a sharp negative high-amplitude deflexion followed by a slower positive wave). S3 and S4 combine to slow wave sleep (SWS, blue) that is characterized by large amounts (>20%) of slow wave activity (0.5–4 Hz) in the EEG. (B) In rats, place cells express a specific firing pattern dependent on the rat's location within its environment. When running through a maze (Run) this leads to a sequential firing pattern that represents the sequence of locations the rat passes in the maze (each row represent one cell, the upper panel shows spikes across time during one lap, the lower panel shows averaged activity). During slow wave sleep (Sleep) after training on an alternation task in the maze the firing sequence is replayed in a time-compressed manner (adapted from Ji and Wilson, 2007). This and other similar experiments are evidence that representations encoded during wakefulness are replayed during subsequent sleep. (C) Recent evidence suggests that the hallmark oscillation of SWS, the sleep slow oscillation (<1 Hz), which is generated in cortical areas, coordinates replay in the hippocampus (accompanied by sharp wave/ripple activity generated in the hippocampus) and plasticity-promoting spindle activity (generated in the thalamus). The coordination of these oscillations enables the gradual transfer of the transient memory representations from the hippocampus to their long-term storage sites within the cortex. The upper trace depicts the unfiltered surface EEG and below, the surface EEG filtered in the slow oscillation and the spindle band are shown (spindles typically occur in the down-to-up transition of the slow oscillation). The inset shows sharp-wave/ripple activity, which is typically nested in the troughs of spindles.