Just stop doing.
Spend 15 minutes doing absolutely nothing. No reading, no brainstorming. No TV or even any music.
There doesn't need to be a purpose -- this really is "nothing".
But just because you're doing nothing, it doesn't mean "nothing" has been achieved. In just 15 minutes your thoughts will have unravelled, stresses eased and focus regained.
By doing nothing, you'll end up being more creative, motivated and focused. Strange, but true.