Sleep disorders are variably prevalent in rare genetic syndromes (1) such as Rett syndrome (RTT, OMIM #312750) (2, 3), a pervasive neurodevelopmental disorder affecting almost exclusively females (1:10,000) and mainly linked to mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) (4). This evidence concerns the gene MECP2 and atypical Rett syndrome.