Sleeps Role In Forming Memories

Sort transient brain events, called "sharp wave ripples," are responsible for consolidating memory and transferring the learned
information from the hippocampus to the neocortex, where long-term memories are stored.

Sharp wave ripples are intense, compressed oscillations that occur in the hippocampus when the hippocampus is working "off-line,"
most often during stage four sleep, which, along with stage three, is the deepest level of sleep.

During stage four sleep, Buzsaki explains, "it's as if many instruments and members of the orchestra come together to generate a
loud sound, a sound so loud that it is heard by wide areas of the neocortex. These sharp, 'loud' transient events occur hundreds to
thousands of times during sleep and 'teach' the neocortex to form a long-term form of the memory, a process referred to as memory
consolidation." The intensity and multiple occurrence of those ripples also explain why certain events may only take place once in
the waking state and yet can be remembered for a lifetime, adds Buzsaki.

http://www.impactlab.com/2009/09/16/direct-evidence-of-role-of-sleep-in-memory-formation-is-uncovered/