We track the march of time in hours, minutes and seconds without much thought. I start my workday at 8AM, head home for the evening around 5 and so on. Throughout the day I work down my “to do” list. Turns out I could, and probably should, be more scientific about how I use my time.

Daniel Pink’s book, “When: The Scientific Secret of Perfect Timing,” optimizes our outcomes by matching our activities with just the right time to perform them. Scientists observing how we perform mental tasks have noticed three things:

Cognitive abilities change over the course of the day.
These intra-day fluctuations can be extreme.
It depends on the task we’re doing.

It’s generally recognized we’re more alert in the morning (unless you’re the 1 out of 5 who is a night owl which pushes your prime time to afternoon) followed by a dip in energy in the afternoon. In fact, scores of students who tested in the afternoon versus those who tested in the morning were consistently lower and were enough lower that it was comparable to missing two weeks of school!

Make note of this: Anesthesia errors were three times more likely in procedures that began at 3PM rather than 8AM.

Once you get through the afternoon (my low point is about 4PM) you get a second wind. And, interestingly, this wind tends to favor creative thinking because, having made it to the end of the day, we’re in a better mood. This, Mr. Pink says, makes for better brainstorming.

What about exercise? It depends on your goals:

Want to lose weight? Morning exercise (before eating) may burn 20% more fat.
Want to boost your mood? Morning cardio workouts can elevate mood throughout the day.
Want to avoid injury? Afternoon workouts take advantage of warmer, looser muscles.
Want to enjoy your workout more? People feel they exert themselves less in the afternoons.
Want peak performance? This surprised me – performance can vary by as much as 26% during the day and the peak is generally 10-12 hours after waking.

How we use our time during the day affects both productivity and happiness – taking breaks is important. Back to the school children who took tests in the afternoon – if they took a 20-minute break to eat, play and chat, they actually outperformed the morning test takers.

While breaks refresh us, researchers say they are most powerful if spent talking with others about random things, not work. And, a walk outdoors beats a walk indoors for a happiness boost.

The takeaway for me is to add some break time to my “to do” list rather than keep my head down from one task to the next to the end of the day. That’s a big change but 30 days makes a new habit, right? We’ll see!