Tony Schwartz talks about this in his article “Breaking the Email Addiction“:

Out of 1200 respondents, some 60 percent said they spend less than two waking hours a day completely disconnected from email. Twenty percent spend less than a half hour disconnected. Email has become our intravenous feeding tube. [...]

It isn’t overload we’re battling anymore, it’s addiction — to action, and information, and connection, but above all to instant gratification.

In the late 1960s, the psychologist Walter Mischel began conducting his famous “marshmallow” experiment. He placed a marshmallow in front of a succession of four-year-olds. Mischel told them they were free to eat the marshmallow simply by ringing a bell after he’d left the room. However, if they were able to wait untill he returned, he told them they could have two marshmallows.

Seventy percent of the children gave up in less than a minute. Only thirty percent were able to wait 15 minutes.

Mischel termed marshmallows a “hot stimulus” — meaning highly seductive — not unlike the ping of an email, or a text.

We’re pulled to anything that provides instant gratification, even when we know we’d get a bigger reward for delaying. We’re also quick to respond to any excuse to stop working on something that is difficult and requires high concentration.

What Mischel discovered is that the low delayers quickly burned down their limited reservoir of will and discipline by staring directly (and longingly) at the marshmallow.

The high delayers found something else entirely to focus on. They never looked at the marshmallow.

Mischel came to call this skill “strategic allocation of attention.” It’s a capacity many of us have lost when it comes to the Pavlovian pull of email.

I don’t want to be in constant respond mode. I want to focus and concentrate on what’s actually important, not be in bondage to the tyranny of the now.

In 2005, a psychiatrist at King’s College in London administered IQ tests to three groups: the first did nothing but perform the IQ test, the second was distracted by e-mail and ringing phones and the third was stoned on marijuana. Not surprisingly, the first group did better than the other two by an average of 10 points. The e-mailers, on the other hand, did worse than the stoners by an average of 6 points.

There is a psychological switching of gears that can require up to 45 minutes to resume a major task that has been interrupted. More than a quarter of each 9-5 period (28%, or 134.4 minutes) is consumed by such interruptions, and 40% of people interrupted go on to a new task without finishing the one that was interrupted. This is how we end up with 20 windows open on our computers and nothing completed at 5pm.

Multi-tasking is dead. It never worked and it never will. Intelligent people love to sing its praises because it gives them permission to avoid the much more challenging alternative: focusing on one thing.

–Tim Ferriss “The Low-Information Diet: How to Eliminate E-Mail Overload and Triple Productivity in 24 Hours” (pdf), p. 5