Regular exercise is supposed to boost a person's energy levels. So why do so many fitness fans complain of feeling fatigued during the afternoon? Making things worse, this workout-induced weariness can make it difficult to stick to a workout regimen.

Researchers and fitness trainers say whether you exercise in the morning, afternoon or evening, small changes in your routine can keep you from sufferingmidday blahs.

Midday is the ideal time to exercise, some fitness experts say. A workout then can give you an energy boost lasting three to four hours, says James McKenna, a professor of physical activity and health at Leeds Metropolitan University in England. A study published in the Journal of Occupational and Environmental Medicine in 2011 monitored 171 employees at a large Swedish public dental-health group who were assigned to an exercise program during work hours. They reported increased productivity and fewer missed workdays.

If you prefer working out in the evenings, it's best to avoid exercising two to three hours before bedtime to avoid sleep disruption, the National Institutes of Health says. On the other hand, if you are a morning exerciser and not getting seven to nine hours of sleep, Lona Sandon, a Dallas fitnessinstructor and assistant clinical nutrition professor at the University of Texas Southwestern Medical Center, suggests getting to bed earlier or exercising in the evening.

To avoid middayfatigue and preserveenergy throughout the day, most trainers recommend doing more moderate workouts, meaning those in which you hit 70% to 80% of your target heart rate. 'Listen to what your body is telling you,' says Ms. Sandon. 'If you have a high-stress work environment then vigorous workouts may not make you feel better. You might be better off with restorative yoga so your brain can slow down.'

An ideal schedule would be two to three high-intensity workouts during the week, mixed in with lighter workouts like yoga, walking or weight training, say fitness experts.

And mix up your workouts throughout the week -- either with cardio, core and flexibility each time, or a rotation of workouts emphasizing endurance, strength and stretching. It not only keeps you energized, it also helps you burn calories throughout the week, says Annie Malaythong, a certified personal trainer in Atlanta who teaches fitness workshops around the U.S. for National Academy of Sports Medicine in Chandler, Ariz.

Nutrition is just as important as moving to keep from feeling fatigued, Ms. Malaythong says. She suggests eating something every three hours, including a snack such as a small piece of fruit an hour before a workout and a meal of protein and carbohydrates within the hour after.

The wrong food can leave you feeling depleted. 'That breakfast pastry or a fast food lunch can sap your energy,' says Samantha Heller, an exercise physiologist and a senior clinical nutritionist at New York University.

And remember to sip water during workouts and throughout the day, says Ms. Heller. 'When you're not hydrated, you will feel fatigued,' she says. 'You may not know why, but you will feel it.'

So why do so many people suffer from the midday blahs?

According to Charles Czeisler, director of Harvard Medical School's Division of Sleep Medicine, the internalbiological clock in the brain's hypothalamus guides daily rhythms of your body temperature, melatonin, blood pressure, sleep and wakefulness.

As the drive for sleep builds up during the day, it produces midday sleepiness for many people, Dr. Czeisler says. It also causes a surge in alertness that peaks in the late evening, he says. Exposure to artificial light in the evening can extend that second wind into the night, making it difficult to fall asleep and deepening the middaytrough in alertness, he says.

Sitting for a long stretch of time can make people feel more tired, says Natalie Muth, a San Diego primary-care pediatrician and health-care solutions director at the American Council on Exercise, a nonprofit that provides fitness certification and training. Moving your body, even for just a few minutes, can get the blood pumping in your body. 'The heart begins to beat faster, delivering more blood and oxygen to working cells,' says Dr. Muth. This increases the release of hormones like endorphins, which not only help people feel good, but also reverses fatigue.