Jean Hagen-Johnson of Raleigh runs three days a week, 52 weeks a year, and always at 6 a.m. She runs during milkman hours not necessarily because she’s a morning person or because she savors the tranquility of a city yet to wake. Her reason is more pragmatic.

“Nothing interrupts a 6 a.m. run,” says Hagen-Johnson. “Do you have a meeting planned? There are no meetings at 6 a.m., no parties to go to. For me, consistency is key.”

Alan Nechemias of Chapel Hill, on the other hand, will work out whenever. He may get up at 6 a.m. to take a mountain bike ride at Umstead State Park, he may leave work at 5 to get in a 35-mile road ride in rural Orange County, he may go to the climbing gym in Durham at 9 at night.

Hagen-Johnson wouldn’t think of changing her workout schedule. Neither would Nechemias. In the overall scheme of things, say the folks who study workout habits, they shouldn’t. Nor should you if you’ve found a time that works for you. But if you have some flexibility, your body may be happier — and more responsive — working out at a specific time.

When’s the best time for you to work out? Let’s start by taking a look at the pros and cons of working out at certain times.

Morning
Pros: Your workout is out of the way before interruptions intervene; You jump start your metabolism, giving you a morning energy boost; You jump start your mind, giving you a crucial advantage over groggy coworkers; In summer, it’s cooler; If you live in an urban area, there’s less air pollution than later in the day.
Cons: It’s morning, your body is still asleep, so is your mind. (Note: There’s been speculation that people who work out in the morning are more susceptible to heart attacks. Research, however, shows that the population in general is more susceptible to heart attacks in the morning, and that people who exercise aren’t at greater risk.)

Lunch
Pros: You defuse stress accumulated during the morning; A workout recharges your mind and body for the afternoon; You’re less likely to eat a big lunch that can result in a mid-afternoon siesta.
Cons: If your boss is a clock watcher, your workout time may be limited; Even if you change clothes and shower, you may still be glowing when you get back to your desk; Clients, bosses, whoever may have other plans for you over lunch.

After work/evening
Pros: Good way to blow off steam after a day at work; You’ll likely eat less for dinner, a meal that can do considerable damage to your waistline since you tend to be sedentary — and thus don’t burn meal calories — in the evening.
Cons: While you may need to blow off steam, you may feel too tired to do so; High potential for distractions (“Hey, Bob, c’mon and grab a beer and some wings with us after work.”); Some people have trouble getting to sleep after a late-day workout; It’s dark and cold in the winter, if you like exercising outdoors.

The perfect time for you. The perfect time for you to workout is the time that’s most convenient, say fitness experts. If you’re not bothered by clients and your boss is out to lunch himself, then noon may be the perfect time to work out. Your muscles have had a chance to warm up and the afternoon boost you get from a nooner is a boon to your productivity, your creativity, your ivity in general. And as carrots go, it’s hard to beat the promise of a 30-mile bike ride, a 5-mile run, or 1,600 meters in the pool to get you through the day. That said, research shows that people who work out in the morning — before the distractions of the day kick in — are more consistent in their exercise programs. What Jean Hagen-Johnson said.

The perfect time, period. Let’s say that time isn’t an issue, that you can work out whenever you want. Lucky you, because when you workout, says the American Council on Exercise, can affect how well you workout. That, says ACE, is because of our circadian rhythms, the daily cycle our bodies operate on. For some, our cycle revs early in the morning, for others, late at night. But for most, our bodies are at peak efficiency late in the afternoon. According to ACE: “It is the influence of circadian rhythms on body temperature that seems to yield the most control over the quality of a workout. When body temperature is at its highest, your workouts will likely be more productive; when your temperature is low, chances are your exercise session may be less than optimal. Body temperature is at its lowest about one to three hours before most of us wake up in the morning, in contrast to late afternoon when body temperature reaches its peak.”

Again, says ACE, when your body temperature peaks can vary. (Your body temperature can vary by 1.5 degrees a day.) If you want to find your peak performance window, ACE suggests taking your temperature every couple of hours for five or six consecutive days.  Chart when your temperature peaks: Your optimum workout window is three hours either side of that peak.

Unless, that is … One caveat to the above. If you’re training for an event — a race, say — that takes place at a certain time of day, try to time your workouts to that time of day. Your body will adjust accordingly and you’ll perform better come race day.