From necessity to sufficiency in memory research: when sleep helps to understand wake experiences

•Activation of neurons expressing learning-related IEG can replace real experience.•Sleep reactivations can be used to create a new memory.•Sleep to study memory: a model where internal representations drive the brain.

Memory is the ability to adapt our behavior by using the stored information, previously encoded. The first investigations of the neuronal bases of the memory trace concerned its properties (location, cellular and molecular mechanisms, among others). However, to understand how this is achieved at the scale of neurons, we must provide evidence about the necessity of a neuronal subpopulation to support the memory trace, but also its sufficiency. Here, we will present past and recent studies that provide information about the neuronal nature of memories. We will show that research on sleep, when cells assembly supposedly carrying information from the past are replayed, could also provide valuable information about the memory processes at stake during wake.