In the middle of the 20th century, adult neurogenesis and rapid eye movement sleep were discovered independently in mammals. The former prompted the idea to apply this rare cellular regenerative capacity in the adult central nervous system to restore damaged brain circuitry, and the latter led to the finding of dynamic brain state changes during sleep and their functional ramifications, including those for memory consolidation. Recently, calcium imaging analysis and optogenetic manipulation have enabled the examination of the activity of specific neuronal populations with precise timing. These technological advances, which allow for the investigation of adult-born neuron activity during sleep in mice, led us to discover its essential function for memory consolidation.