The most common question regarding this would be:  

Which is better, training in the morning or in the afternoon?

That's a very interesting question, and the answer for it is quite simple:

What suits you the most.

Both evening and morning exercise have cons and pros that I will review next, but the best time to train for anyone is the best time to fit on their schedule, this way it's hard to miss a workout.

Training in the morning

I will start with Morning exercise, there are many benefits from doing your exercise first thing in the morning, most importantly: studies have shown that training first thing in the morning is better habit making than later in the day. And that's very important, making exercise as a habit is a key to a better healthy life.

It’s also much easier to schedule, you can always wake up an hour earlier to exercise, but training later in the day is a little harder as you never are sure of what will come up.

Another benefit with morning exercise that it’s the best for fat loss, in the morning after all time fasting asleep, your blood sugar is at lowest so the body will start burning the fat to supply the energy needed to exercise, heart rate and metabolism will increase and help you burn more calories throughout the day while feeling lots of physical energy.
You can still mimic this if you train in the afternoon, you just fast a few hours before your workout and the effect would be similar, but make sure to eat a lot of protein after wise as the body may also metabolize amino acids for energy.

A Number of studies also have shown that morning exercise helps improve sleep, which in turn helps control appetite in some hormones chemical reaction, so we would have better control on what we eat.
Lastly, if workouts are done outdoors then morning exercise have the benefits of cooler temperatures in the summer and less air pollution, so more oxygen for your body improving your workouts.

These are very good advantages for morning exercise, however I must mention the cons as well:
Since body temperature is at its lowest before awaking, this means lower energy and blood flow in the morning, which require more warming up to avoid injuries on cold stiffed muscle , be sure to warm up good and do some stretching as well.
Another thing is some people are just not morning people and may do anything for another hour of sleeping , we know how tempting sleeping can be for most of us, how many time you press snooze on your alarm? I know I do at least a few times, sleeping just tempting for many which make it hard for many people to do.

Afternoon Training

A number of studies have shown that afternoon workouts are more efficient than morning ones when it comes to performance, performance is greater in the afternoon than in the morning, that's due to a number of reasons. The body metabolism is more active later in the day, the muscle are more warmed up and stretched, and many hormones levels are high which helps improve performance.

Since muscles are warmed up during normal day activity, fewer injuries occur in afternoon workouts.
Afternoon workouts also help relieve the stress you had during the day, which in turn improves your mood.

As for cons for afternoon workout:
There are many things to distract you later in the day than in morning, you may have to choose between going to the gym or going out with friends, watching a movie or excreting sweat!! so it takes a stronger commitment to plan your workout late in the day.
Also avoid training late at night as you may have trouble sleeping.

This comparison between morning and afternoon gives you a hint of which is the best time for you.

Circadian Rhythms

We all have an internal biological clock inside us, adjusting to our way of life, it’s called circadian rhythms/cycle, this cycle manage our different body functions and regulates them all the time, such as body temperature , blood pressure ,heartbeats, and a long list of other physiological functions. It all affects our body activity and mental alertness.
Now this Cycle works fulltime 24 hours a day, there is a normal one that we are born with , but based on our behaviors and otherenvironmental things around us it can change.

The illustration shows how a normal biological hour is.

How to apply this, think about last time you had a new work, changed your shift from early time to late time, or last time you had to stay up very late or wake up very early. You would see that it affected your body and your state of mind a lot, that’s because you body is not expecting such thing from you.

Let’s say you read this article and decided it’s better for you to do morning exercise, you set the alarm tomorrow and wake up early to do so, you probably will have a hard time waking up not to mention finishing your workout, but since our body is an incredible machine that adjust to many things, next time you wake up for your exercises it will be easier and as you stay on that schedule, eventually it will be as easy as the afternoon exercises was for you.

What happens here that your Circadian Rhythms cycle was reset according to your new behavior.
This is also important if you are preparing for an event, let's say you have some kind of sport event in the morning, then the best way is to start training at the same time the contest will be held, to allow you body to adjust so you can work on your full potential at the time of the event.
We must not forgot that as human we are all different, so for the internal clock to reset and adjust to the new behaviors, its different for every individual , the adjustment may take a short time for some and more than a month to other.
So after deciding what timing is best for you, make sure to at least give your internal clock a month to reset, it may hard at the beginning but it will get easier as you move on.

Now that we've discussed both pros and cons for each time, and the way our internal clock works, you can choose the timing that fits you the most and make your body adjust to it and stick without.
And remember it's not about the timing, it's about making exercise part of your life, accompanied by healthy eating and you are doing your body right.
Good luck

Summary

Morning training

Pros:

• Habit forming, making people stick with it more.
• There is always time in the morning, just wake up earlier.
• For outdoor training, pollution is at its min, and the air is better oxygenated.
• Improve you physical and mental activity for hours.
• Increases metabolism, and great for fat burning
• Turn helps control appetite

Cons:  

• Needs longer warming up and stretching.
• Easier to get injuries with blood flow low to the muscle and tendons.
• Some people are just not morning persons and too lazy to get up. 

Afternoon training
 

Pros:

• More efficient training and better performance.
• Muscles are warmed and stretched later, so less injuries.
• Helps relieve the stress you had during the day. 

Cons:

• Too many distractions may occur.
• Very late training may make it hard to sleep.
• Less fat burning compared to mornings.
   

Shady Eid is a certified personal trainer, writer and competitive bodybuilders based in UAE, for questions and comments you can contact him at: Shady4pt@yahoo.com