You open that video on YouTube, watch it for five minutes and then switch to Facebook to check for any updates. The cycle continues and you soon realize that you wasted an hour at work Wilfing while the task at hand (the status report) remains incomplete.
Sites like Facebook, Google News or Twitter are certainly not bad for health but our constant urge to check sites for new content certainly distracts us from actual work leaving us less productive and at times, inefficient.
There's help at hand. You can either temporarily block websites or you edit the HOSTS file in Windows and permanently block websites that you think waste your time.
Then there are extensions like LeechBlock that will prevent you from browsing websites that you alone specify. It's like a self-imposed restriction to help you concentrate on work. You can specify the time periods and even the days of the week when access to particular sites should be restriced.
Sites to block can be specified using wildcards (e.g., *.blogspot.com) and exceptions (e.g., +labnol.blogspot.com).