User blog comment:Omega natsu2/Chapter 365: Fairy in the Jail!, Predictions./@comment-112.203.211.177-20131219235849/@comment-112.210.232.254-20131220182626

Crawford said that he used "Chamomile" to produce those herb tea, Chamomile is a daisy-like plant that is used to produced herb tea that induced sleep and knowing Erza and Mira are knowledgeable mages I don't think it's far fetched to think that they knew what Chamomile is thus be alerted not to drink it, and thus are just faking their sleep to know Tartaros' HQ and info about "Face," though this is just my theory