The early bird gets the worm — that's the mantra the 5-a.m. gym-goers live by. However, exercise improves sleep quality, so why not work out later in the day, closer to bedtime? There's been a long-standing debate over the optimum time to work out, due to the physiological effects exercise has on the body. So, no matter what time you choose to work up a sweat, consider the pros and cons of a.m. vs. p.m. workouts.

Pros for A.M. Workouts

Working out in the morning can boost energy for the rest of the day. An a.m. workout jump-starts metabolism in the same vein that eating breakfast does. The thermal effect of exercise (or activity) lasts at least four hours after a workout. That increases the total calorie expenditure throughout the day.

In the 8-12 hours prior waking, your body was in sleep mode and, depending on the quality of sleep, in a process of regeneration. A workout causes microscopic tears in the muscle fibers. Oxidation also takes place on the cellular level and energy stored in the muscle fibers are utilized. Sleep is the time for the body to repair damage. Antioxidants repair cellular damage. Food is digested and converted to glucose. In the morning, you're using a fresh, repaired and, in some instances, new body. During REM sleep, dreams are also a chance for the body to rehearse or even learn new motor skills — that clean and snatch is smoother and easier than it was during your last workout because you essentially had time to practice during your sleep.

Hit the Snooze Button

So the early morning dew isn't enough to get you to roll out of bed, and that's OK. What works for a person who gets enough rest every night and has enough ATP and muscle glycogen stored up won't work for the person whose last meal was at 4:00 p.m. the previous day and went to bed after midnight. Your body has been in a state of fasting — or even starvation, depending on your eating habits — since the night before. Not to mention that an early morning workout may wear you out for the rest of the day, especially if you didn't get the recuperative sleep you needed. That groggy feeling in the morning will have you leaning toward an evening workout in the a.m. vs. p.m. workout face-off.

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Pros for P.M. Workouts

Muscle strength is dependent on the muscle's ability to contract as well as its tensile strength — the ability for the muscle to stretch. Think of your muscles as rubber bands: An old, dry rubber band cannot stretch. If you try to use it, it snaps — this is sometimes how your muscles will feel first thing in the morning. Later on in the day, your body is more warmed up. That results in the ability to work out longer and lift heavier weights, and it means you are less likely to injure yourself. Your joints also benefit from being warmed up: Synovial fluid fills every joint and acts as cushioning and lubrication. First thing in the morning, that stiff feeling in your knees, ankles, hips, or shoulders is caused by synovial fluid that's viscous. Later on in the day, your body is like a well-oiled machine.

Hopefully you've had your five to six small meals throughout the day, and we all go through those days when you're celebrating a coworker's birthday or the company has lunch catered for the entire office and you eat a little more than usual. The calories you've consumed throughout the day are the perfect fuel for an evening workout.

Pillow Talk

When choosing sides between a.m. vs. p.m. workouts, it's important to remember a few things: Evening exercisers often do so because they want to be “tired out” by the time they hit the sack. However, the thermal effect of exercise is the same at any time of day; body temperature rises during and after physical activity. So the body will be "alert" when you're trying to sleep. A study published in the journal Sleep found that a dip in body temperature took place two hours prior to subjects falling asleep. The decrease in body temperature is the body's cue to relax and then ultimately fall asleep. Working out close to three hours before your bedtime can disrupt the body's circadian clock and may affect your ability to fall asleep and stay asleep.

Timely Fitness

At the end of the day (or the beginning, as the case may be), the time you choose to exercise is the time that's best for you — when you can fit it into your schedule. Making time for physical activity in the grand scheme of things is what's really important to keeping you fit and healthy, whether you do it at 4:00 a.m. or 11:00 p.m.