When Is the Best Time of Day to Do a Cardio Workout?

An extremely common question that comes up in any discussion of workout program design has to do with timing. The topic gets especially controversial, though, when you start talking about cardio.


So, when is the best time of day to do a cardio workout? What does the research show?


Granted, it might sound absolutely horrid, but a solid body of scientific evidence suggests that there are some pretty fantastic reasons to knock out your cardio workout first thing in the morning.


In fact, much of this research surrounds something called “fasted cardio” - meaning that you'd work out before you even had breakfast.


What are the benefits of fasting cardio and morning workouts in general?


Enhanced Focus and Energy


Although we don't often think about it when it comes to workout design, the human body naturally goes through several hormonal changes throughout the day.


These can have a major impact on our athletic performance and on the efficiency of our workouts.


Close up of a women's shoes while walking on a treadmill


When you first wake up, your body naturally produces high levels of both adrenaline and cortisol. Both of these hormones enhance mental focus, energy levels and fat metabolism.


You may associate cortisol with fat gain but this only happens when concentrations of the hormone are too high for too long. Keeping these hormones elevated when you head out for your morning cardio could be a major advantage.


This hormonal response, though, is stunted once you eat. Early morning cardio also has the advantage of taking place before the other stresses of the day crowd in on you and start demanding your attention.


Increased Fat Metabolism


As mentioned, the hormonal condition present in your body first thing in the morning will increase the amount of fat that you burn for fuel. But that's not all.


Generally, carbohydrates are your body's favorite source of fuel, especially during cardio. When you sleep, though, your body continues to burn carbs to recover from the day and support your standard biological systems.


When you wake up after that eight hour fast, your carb reserves are tapped out. So, your body has to look else for fuel. And fat is the solution.


According to a 2013 study from the Northumbria University, fasted cardio could increase your fat metabolism by as much as 20 percent.


Cropped image of a woman working out on an elliptical machine in a gym


What About Catabolism?


There's a pretty strong chance, though, that you've heard this all before. And, if you have, then you've also heard the counterargument: fasted cardio causes your body to destroy your muscles for fuel. And this is true. Maybe.


It all depends on the type of cardio that you do. The process of breaking down your muscles, called catabolism is not ideal and your body tries to avoid it as much as possible.


This is for two reasons.


First, muscle is hard to build and important to your overall function. Your body wants to protect it.


Second, converting protein to usable carbohydrates is a pretty complicated, time-consuming procedure. It's not very efficient.


So, catabolism will only happen to a noticeable extent if there isn't enough fat present to support your workout. The trick, then, is to make sure that you never pass that threshold.


And keeping the intensity of your workouts high is the way to get that done. Rather than sticking with classic (boring) steady-state cardio, go with high intensity interval training (HIIT).


Not only does HIIT burn more calories from fat in less time but it also tends to spare your muscles more than steady-state.