On the other hand, sleep spindles (especially associated with N2 NREM sleep stage, but can also occur during N3 NREM sleep stage) are also crucial for declarative consolidation; indeed they are enhanced (increasing in density) after declarative learning, their increase is associated with a better memory performance (which has been proved using pharmacological manipulation of spindles' density, and measuring outcomes on learning tasks). A working model of sleep and memory stabilization Schreiner and Rasch (2017) proposed a model illustrating how the cueing beneficial effect on memory during sleep could function, which includes theta and gamma waves and sleep spindles. Increased theta activity represents the successful reestablishment of the memory after the cueing: if such an increase is observed, it means that the association between the cue and the memory trace is strong enough, and that the cue is presented in an effective way and time. Theta waves interacts with gamma activity, and - during NREM - this oscillatory theta-gamma produces the relocation of the memory representation, from the hippocampus to the cortex. On the other hand, sleep spindles increase occurs right after or in parallel to the theta augmentation, and is a necessary mechanism for the stabilization, the reinforcement and also the integration of the newly encoded memory trace. Importantly, in this working model, slow oscillations have the role of a 'time-giving pace maker', and seem to be a prerequisite for the success of cueing. According to this model, enhancing only slow waves or only spindles, is not sufficient to improve memory function of sleep: both need to be increased to obtain an influence and this latter. NREM in other animals Not much is known about NREM, so scientists have conducted studies in other animals to potentially understand more, in particular why the brain has evolved to have two distinct states. In their studies, it was found that between birds and certain mammals like dolphins, their brains exhibit similar behavior. It was found that certain species of birds have half their brain's hemisphere release brain waves similar to a human's during NREM sleep, and the other half of it fully conscious, allowing them to fly while sleeping. Certain species of dolphins also exhibit similar behavior as birds in order to be able to swim while sleeping. In rats, after a 24-hour sleep deprivation, it was found that there was an increase of slow-wave activity in NREM sleep, which corresponds directly