People just aren’t good at multitasking. Not only are multitaskers bad at filtering incorrect information but, on average, multitasking drops accessible IQ by 10 pts. In 80 clinical trials, Dr. Glenn Wilson, discovered that workers who juggled messages and work suffered the equivalent of missing a night's sleep and up to double the 4-point drop seen after smoking marijuana.
What can you do? Protect your state of creative flow. Do one think at a time. Prioritize in the morning. Don't use IM or auto notifications while working. Fight the urge to compulsively check email, Facebook and your phone.
Another bummer: Listening to music while working reduces productivity when the music has lyrics and your work relates to language (like speaking or writing emails).
On a positive note, naps boost productivity.