It's not the hours you put in - it's what you put in the hour
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It's really annoying if you see a lot of people at work watching Youtube, using twitter, writing private emails. It ultimately wastes a lot of time and money of your company.
One solution is to simply forbid watching youtube, using twitter, and everything else.
Such restrictions will probably work in a factory setting. But it will not work in a creative environment such as IT.
If you put restrictions on people they will not like it. They will underperform or even leave your company. If you want to change something work out the problems with your people. They are clever and know what goes wrong. If everybody agrees on a solution it will be implemented. If you force a solution or restrictions you will fail [2].
You can never expect that anyone can work 8h fully concentrated on a complex problem. Just does not work. People have certain flow phases where a lot of stuff gets done, and some phases where nothing gets done. It's just plain human [1].
The truth is:
If your feeling is that half of your company is made up of youtube watching zombies - most likely it's your problem as leader (CEO, CTO, project manager etc). Motivate your people, give them meaningful projects a silent environment and they will perform. Give them stupid projects, treat them like stupid little kids and they will become youtube watching zombies.
[1] Book Peopleware, Tom DeMarco und Timothy R. Lister
[2] http://ars-machina.raphaelbauer.com/2011/05/top-down-approach-of-management-recipe.html