1. Morning Roadblock: YOU'RE SLEEPY

WHAT'S HAPPENING

"The predawn hours are challenging because body temperature and heart rate dip to their lowest point at this time," Moffitt says. "In the presence of light, body temperature and heart-rate increase, which makes it easier to be active." The carbohydrates in your last meal play a role, too. If you skipped dinner or ate fast-digesting carbs like rice, bread, or sugary desserts, your glycogen levels will be depleted, making it even harder to muster the energy to get up.

HURDLE IT

Prep for an early-morning run the night before. Eat slow-digesting carbs like broccoli, beans, and lentils. Set your automatic coffeemaker to brew before you wake. "Caffeine can help stimulate your arousal system and get you ready to run," Moffitt says. Shut-eye is important, too. If you're constantly waking up feeling exhausted, it's a sign you aren't getting enough z's. Turn off the computer and TV at least 30 minutes before you hit the sack and get blackout shades for your windows—the absence of light boosts production of melatonin, a hormone that makes you feel sleepy.

Midday Roadblock: WORK DEMANDS AND HUNGER PANGS STRIKE

WHAT'S HAPPENING

Melatonin production is at its lowest around noon, so physiologically that's when you're most alert, Moffitt says. That means you may be motivated to continue plugging away. And your blood sugar dips, which may make you too famished or weak to run.

HURDLE IT

Schedule your run like you would any other meeting, then record your time and distance to give you a sense of accomplishment, says Steve DeVries, Ph.D., professor of exercise psychology at Cornell College in Iowa. Try splitting your lunch in two: Eat half an hour before you run, then have the rest afterward. If you're still tempted to bow out, consider this: A British study found that midday exercisers increased their productivity.

Late-day Roadblock: IT'S BEEN A LONG DAY AND YOU'RE BEAT

WHAT'S HAPPENING

Mental fatigue lowers dopamine, a brain chemical that makes you feel energized. Plus, blood sugar dips again between 5 and 7 p.m.

HURDLE IT

Don't confuse mental fatigue with physical fatigue. Researchers from Bangor University in Wales report that short-term mental fatigue doesn't impact the physical function. So it's your mind—not your body—that craves downtime. And running is the perfect antidote. "Running elevates your nervous system, which increases your sense of alertness," Moffitt says. Resist the urge to skip your run by packing your gear, changing at work, and going directly to the gym or trail. Even better, join—or start—an after-work running group. Keep your energy up with a snack before your run.

Midrun Roadblock: YOU'RE TEMPTED TO WALK OR CUT YOUR RUN SHORT

WHAT'S HAPPENING

On a run that's longer than 90 minutes, it's probably a fuel issue. "As you run out of carbohydrates, your body uses fats for energy, which takes longer and leads to a sluggish feeling until that fuel kicks in," Moffitt says. Or it could all be in your head: "You may feel like you've done enough, and no one's watching anyway," DeVries says.

HURDLE IT

On runs longer than an hour, consume 100 to 200 calories every 30 to 60 minutes to replenish energy stores, says Nancy Clark, R.D., a sports-nutrition expert. Keeping a running log can also discourage you from taking shortcuts, DeVries says. You won't want to write that you did eight miles instead of 10.

Postrun Roadblock: YOU WANT TO CRASH, NOT STRETCH AND ICE

WHAT'S HAPPENING

When you stop running, your blood pressure drops and blood pools in your legs, which may make you feel weary and lightheaded. "You may feel 'my work is done; I deserve to chill out now,'" DeVries says.

HURDLE IT

Slow your pace the last mile of your run, and then walk for a few minutes before stopping. Because muscles are most pliable and inflammation is at its peak right after running, try to stretch and ice trouble spots after cooling down. Then, within an hour of finishing, consume a carb-protein snack. "Carbohydrates refill your glycogen stores, while protein repairs muscles," Clark says. Record your postrun routine (stretching, icing, refueling) in your log so you'll consider it just as important as the run itself, DeVries says.

Adjust Your Running Clock

How to get over three common tricky timing issues

Are you usually a morning runner, but winter's dark mornings have you hibernating?

Instead of getting dressed half-asleep by the dim glow of a nightlight, put on your running clothes in a brightly lit room. When light hits your eyes, it signals your pineal gland to stop producing melatonin, a hormone that makes you feel sleepy. Compact fluorescent light bulbs (CFLs) work best because they closely mimic natural sunlight.

Are you usually an evening runner, but your key race is in the morning?

Run at the time of your race once a week. Train your intestinal tract by rehearsing what you'll eat. Eat your normal breakfast (about 500 calories) four hours before your race time, then snack on a banana or energy bar an hour before you head out. If prerace jitters make you too nervous to eat the morning of a big event, practice eating breakfast the night before.

Are you usually an evening runner, but have to run in the morning because of schedule conflicts (or vice versa)?

Give the transition at least two weeks and don't be surprised if you don't feel or run your best for a little while. It takes a couple of weeks to fully adjust to a time change. Switching your workout routine may make you feel like you're recovering from jet lag on a run. But your body will adapt to the new schedule.