For all the gadgets I've gathered around me and despite for 18 months doing everything on a screen through a keyboard or touchpad, I know have a whiteboard and put sheets of paper on the wall. I take notes. And when I need to hunker down I pick one of these and commit.
Green - One Minute
Yellow - Three Minutes
Black - Thirty Minutes
Blue - An hour
I practice I find 3 minutes does the job and I may keep going for several hours. My heart is telling me not to panic then the adrenalin rush kicks in.
An hour is just that. It is a pre-arranged slot and I find I am yawning at 55 minutes, as if I am waiting for the class to end.