It seems that central to the hypothesis of Chiara Cirelli and Giulio Tononi (specifically, that we need sleep to prune synapses)1 that all synapses grow during the day, whether they're stimulated by specific activities or not. I didn't see anything in the article that specifically supports the idea that all synapses increase in strength during the day—has any research been done on it?
Regardless, it seems that some kind of combination of Cirelli and Tononi's theory and the predominant theory that we need sleep to replay and consolidate memories may make sense. Specifically, sleep may be needed to prune synapses, and a replay of memories may be needed to prevent new connections from being pruned at that time.
Without replay, new synapses may be pruned, preventing retention. Without synapse pruning, replay wouldn't be needed to maintain new connections.
If this is the case, then Cirelli and Tononi's main hypothesis is ...
