This article on sales trends for 2013 identifies Technology Fatigue.

“While technology has profoundly improved the effectiveness of salespeople, there is an interesting trend that has been growing over the past couple of years and will only get worse. Salespeople today are always available to customers via their cell phones, constantly receiving a gargantuan stream of e-mail information, and every account update they make in Salesforce.com is broadcast to sales management. Many long time salespeople are suffering from “Technology Fatigue” and complain they are burning themselves out.”
http://blogs.hbr.org/cs/2012/12/top_10_sales_trends_for_2013.html

If this is you it’s time to teach your clients how to be with you. Set clear boundaries and be consistent. The reality is that once the client understands that the best way you can serve them is to be on your game during reasonable business hours, they might in turn take the lead from you and start doing the same.