A study showed that people distracted by incoming email and phone calls saw a 10-point fall in their IQs. What's the impact of a 10-point drop? The same as losing a night of sleep. More than twice the effect of smoking marijuana.
Doing several things at once is a trick we play on ourselves, thinking we're getting more done. In reality, our productivity goes down by as much as 40%. We don't actually multitask. We switch-task, rapidly shifting from one thing to another, interrupting ourselves unproductively, and losing time in the process.