Sleep on it, but only if it is difficult: effects of sleep on problem solving
Memory & Cognition (2013) - Comment
pubmed: 23055117  doi: 10.3758/s13421-012-0256-7  issn: 0090-502x  issn: 1532-5946 

Ut Na Sio, Padraic Monaghan, Tom Ormerod