The burden is on the receiver to make sure that they are undisturbed, if that's what they want. I've never given it a second thought about when I was sending an e-mail, and I say this as someone who could be awakened by a off-hours e-mail, if I didn't take steps to silence my electronics when I don't want to be bothered. This may have come from the fact that I worked for a multi-national, with colleagues in nearly every time zone on the planet. No matter when I send an e-mail, it's the middle of the night for someone. If anybody had griped about getting an e-mail in the middle of the night, I would have given them a (virtual) raised eyebrow and not changed a thing. There's no way that I would stay up late, or get up early, or try to customize the delivery for the convenience of every recipient. My colleagues are all adults and supposedly can manage their own lives.