Best time to send marketing email

Pages and pages have been written about the best time to send email. Marketers spend significant amounts of energy discussing and researching the best time of the day and the best day of the week to send email. I have long thought that these discussions do not put enough attention on individual end users and how the recipients interact with email.
Researchers recently developed a model for email user behaviour that splits email users into two classes “e-mailaholics” that send, and presumably read, email all the time and “day labourers” that send, and presumably read, email during standard business hours. There is very little transition between groups, 75% of users stayed in the same usage group over the 2 years of the study.
What does this mean for senders? Senders need to know know how their recipients use email and which user group recipients are. By analyzing clicks and opens, senders can classify recipients and use that data to send mail that is more relevant and better targeted.
h/t arXiv blog at Technology Review