I always advise my clients to do cardio when they feel the best time to do it. Meaning, listen to your body and try to stick with that time.
With that being said, there are a few principles that I️ have them follow.
Don’t do cardio on an empty stomach. If your schedule only allows you to do cardio first thing in the morning, I️ recommend having something light 30-40 minutes prior. With this, you will be able to perform better and preserve muscle mass in the process.
Ideally the best time to do cardio is right after a weight training session or separate from your weight training. By doing it right after, you will have more energy to push through your workouts since cardio burns more glycogen than weight training depending on the intensity.
Also, a hard cardio session can lower protein synthesis while protein breakdown goes up. Doing cardio on separate days than weight lifting ensures that you have the proper energy to perform your best in either your cardio or lifting session.