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. Yeah, that's right, the thing most job applications insist you be good at decreases your efficiency more than weed. Tell that to the folks in HR.
So 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. Carve out time to work distraction-free.
Here's a simple formula to freak you out. At best it takes about two minutes to recover from an interruption when you're in a state of creative flow. It doesn't matter how small the interruption is - could be looking at a Facebook notification, glancing at email or someone tapping you on the shoulder. Longer interruptions take more time to recover from. Lets assume you receive 20 small distractions in a single day. 20 x 2 = 40 minutes of lost time per day. That's 200 minutes per week, or a little over 3 hours. That's almost half a day that you could've spent on a personal project. Multiply that across the year and you've lost 160 hours. That's a month of 40-hour work weeks. Imagine what you could accomplish if you had a month of extra time to get things done.
To make things worse: some studies suggest it takes up to 24 minutes to recover your state of flow.
On a positive note, naps boost productivity.