Email messages that are sent at five or six in the morning according to Dan Zarrella had the highest click-through rates (CTR). This was based on statistics from a database provided by MailChimp of nearly 10 million email sends.

It also helps if your content is personal, relevant, and delivered timely.

Reference: Zarrella’s Hierarchy of Contagiousness: The Science, Design, and Engineering of Contagiousness