When cardio is on your workout plan, it truly doesn't matter if you do that workout in the morning or the evening — at least in terms of long-term performance. A 2012 study published in the Journal of Strength and Conditioning Research reports that the effect of time of day on aerobic performance is insignificant.
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Time of day matters for strength, flexibility and power training, however. Later in the day, your body temperature is warmer and you perceive exercise to be easier, especially when it consists of the really hard lifts on the gym floor.
So what makes some people almost religious about their morning routines? A lot of it has to do with preference, but they may also be more likely to be committed to sticking with an exercise plan. If you do your cardio first thing in the morning, you're more likely to get it done before the day gets the better of you.
Read more: Definition of Cardio Exercise
Benefits of a Morning Routine
The No. 1 reason many people work out in the morning is to guarantee it happens. A work day that runs late, tempting invites to happy hour, after school activities or simple loss of motivation are less likely to get in the way of your calorie-burning routine. Working out in the morning also makes your more likely to stick with fitness in the long run.
If it's your habit to wake up in the morning and get your 30 minutes of cardio done, you may also feel more satisfied. Many people feel rejuvenated after a brisk walk or lap swim and ready to hit their day.
Morning cardio also has its benefits if you're training for a specific event, such as a 5K or a mud run. These events tend to have early starts. When you train in the morning, you're priming your body to be prepared to perform at a similar time so you're more ready for race day.
In the morning, you also don't have food getting in the way of exercise. Evening cardio that happens after a burrito lunch or garlicky pasta dinner may be tainted by repeats of these delicious entrees. Morning pre-workout food usually consist of relatively bland, high-carb foods, such as a banana or a plain toasted bagel. You'll benefit from the energy of the food without any potentially offensive digestive consequences.
Read more: The Best Morning Exercise Routines
Evening Exercise Effects
If you hit snooze one too many times to make it to the gym, don't despair. Evening cardio still burns calories and may just feel easier. Your heart and muscles are already warmer and more prepared for exercise than they are in the morning. This means you spend less time (but not no time) warming up and have a lower risk of injury.
If you've been careful to eat well during the day and not take in an excessively fatty or spicy lunch or pre-workout snack, you're also better fueled for cardio in the evening.
Scientists once thought that evening cardio would keep you up all night — but 2014 research00045-8/abstract) published by the National Sleep Foundation found this just wasn't true. Exercise at any time of the day improves sleep quality according to more than 1,000 respondents in the Foundation's study.
If you've got a high-intensity interval training cardio session on tap, you might even be better off saving it until later in the day. High-intensity interval training involves alternating bouts of very intense work performed at 90 percent or more of your maximum heart rate with short bouts of rest. Because your body is warmer and better fueled, you'll likely get more out of this session if it's performed in the evening.