Sleep spindle refractoriness segregates periods of memory reactivation

The stability of long-term memories is enhanced by reactivation during sleep. Correlative evidence has linked memory reactivation with thalamocortical sleep spindles, although their functional role is poorly understood. Our initial study replicated this correlation but also demonstrated a novel rhythmicity to spindles, such that spindles are less likely to occur immediately following other spindles. We leveraged this rhythmicity to test the role of spindles in memory by using real-time spindle tracking to present cues inside versus outside the presumptive refractory period; as predicted, cues presented outside the refractory period led to better memory. Our findings reveal a previously undescribed neural mechanism whereby spindles segment sleep into two distinct substates: prime opportunities for reactivation and gaps that segregate reactivation events. One Sentence Summary The characteristic timing of sleep spindles regulates when memories can be reactivated during sleep.