• sources: BBC article and cracked.com article

Procrastination is connected to the reward system in the brain.

Dopamine motivates us to seek out solutions to satisfy goals, i.e. work. And the closer we think we are to earning a reward, the more dopamine our brain releases. And, conversely, if we think a reward is a long way off, dopamine doesn't get released, and our motivation is squelched.

However, if dopamine receptors are blocked, we just work and work and work, with each task carrying the same weight. We know this because scientists injected a dopamine-blocking DNA construct into the brains of monkeys, as scientists are apt to do, and the DNA totally changed the way the monkeys worked. (See: DNA targeting of rhinal cortex D2 receptor protein reversibly blocks learning of cues that predict reward)

Before these monkeys got the dopamine blocker, they were trained to read visual cues that told them how many more tasks they'd have to complete before getting a reward. Their performance improved as they got closer to their reward. But after the monkeys' dopamine receptors were blocked, they could no longer judge when the reward was coming, so they just kept working steadily.

This means that, if you're working on a task that has a far-off or questionable reward (schoolwork), and you think of something that has a more immediate reward (checking facebook), you'll get a jolt of dopamine that'll motivate you to switch over to facebook.

And, unfortunately, when the task at hand is schoolwork, almost everything has a higher reward. ("OMG I FORGOT TO WASH THE DISHES. OMG I FORGOT TO LOOK OUT THE WINDOW FOR 10 MINUTES.")

Amphetamines like ritalin make it easy to focus on one task without getting distracted because they alter the way dopamine functions, making everything appear equally rewarding.