Figure 1. The Stages of Sleep and Memory.

A. Hypnograms demonstrate sleep architecture in humans (upper panel)_and mice (lower panel). In mammals, sleep is classified as rapid eye movement (REM) sleep and non-rapid eye movement (NREM)) sleep. In humans, NREM is further subdivided into 4 stages (corresponding to increasing depth of sleep), with Stages 3 and 4 often collectively termed “Slow Wave Sleep” (SWS). NREM and REM progress in cycles, varying in length across species. In humans, sleep is organized into a single nocturnal bout with approximately 90-minute NREM-REM cycles. SWS dominates the early night, while Stage 2 and REM sleep dominate later in the night. In contrast, rodent sleep is organized into many short bouts. Compared to humans, rodent NREM-REM cycles are compressed, lasting approximately 15 minutes. Both humans and rodents demonstrate periods of microarousal indicative of short arousals to waking EEG activity that do not grossly disrupt sleep architecture. B. During NREM, cortical EEG “slow waves” (0.5 – 4 Hz) result from cell populations switching between hyperpolarized “down-states” and depolarized “up-states”, accompanied by synchronized phasic “sleep spindles” (<2 seconds, 11 –15 Hz in frequency). At the level of the hippocampus, high-amplitude “sharp wave” deflections originating in the CA3 subfield give rise to corresponding fast “ripples” in CA1, which collectively form sharp wave ripple (SWR) complexes (~200ms, 100 – 250 Hz) [143]. SWRs are temporally coupled to the slow wave and spindles in neocortex, forming a potential oscillatory substrate for hippocampus-neocortex communication during NREM. During REM, both the neocortex and the hippocampus express faster, tonic, patterns of theta activity (~ 4 – 8 Hz, varying in frequency across species [130,144,145]). C. Memories develop in several stages over time. A memory is first encoded by engaging with an experience. Resulting in a neural “memory representation”. Following encoding, this representation can undergo several stages of development [1,3], the most commonly recognized being consolidation wherein memories become increasingly resistant to forgetting overtime. Following successful consolidation, memories can be retrieved long-term. Both sleep before, and after learning seems to be modulated by, and capable of modulating, these distinct phases of memory.