Are you determined and optimistic in the morning, and then a lazy Scrooge by lunchtime? According to new psychology research, that seems to be a pattern for many people.

A study shows we tend to be more stressed and less productive later in the day.

The findings suggest self-control is a quality that fades as the day goes on, and eventually, people give in to temptation and frustration.

"The reason that we're more productive in the morning is that we're more free from distraction," said Luke Iorio, president of the Institute for Professional Excellence in Coaching, based in Shrewsbury. "We're able to stay on task."

That positivity tends to wane when reality sets in: there are emails waiting for responses, co-workers wanting opinions and family members needing help.

"They're getting distracted by everyone else's agendas," Iorio said. "They're getting distracted by other situations that bring up stress for them."

Throw someone off their game, and the domino effect begins. A spike in stress means a drop in energy and decreased willpower. All of a sudden, slacking off sounds better than finishing that big project at work. A bag of chips sounds better than that strict diet you were hoping to follow.

The best way to optimize performance, according to Iorio, is working in "sprint mode." Focus hard on your task for a solid 70 to 95 minutes. Then, take a break to regroup and start the sprint again.