What's better for building muscle: morning workouts or evening ones?

It’s a common question wondered by gym-goers everywhere: Does when you work out really matter for muscle gain? Are there really any benefits to getting your heart pumping first thing in the morning? Or is it actually better to work out after your body has had the chance to fuel up throughout the day?

Like many questions in the fitness field, there doesn’t seem to be a clear-cut answer. Still, there has been some research on the topic, and the evidence seems to favor later workouts. In fact, a 2016 study from Finland found that a combined program of strength and endurance training could lead to greater gains in muscle mass when performed in the evening rather than the morning.

An earlier study looking at upper body and lower body power output in the morning vs. the evening found something similar: There was greater muscle strength and power later in the day. But, the researchers found, when people ingested caffeine before the morning workout, that raised their performance to the levels seen in the evening.

So, what gives? When should you really workout to get the greatest gains?

It all depends on your unique schedule and preferences. If you're trying to build muscle, there are different perks to working out in the morning or at night—but the pros are more about your how your personal schedule and habits correlate with the time of your workout, rather than the time of day itself, says Menachem Brodie, C.S.C.S., an exercise physiologist and trainer at Human Vortex Training.

One perk to working out in the morning is that you often end up fitting in more workouts over the course of the week—and the cumulative effect can lead to greater gains, he says.

"A benefit to working out in the morning is that you get the workout over with early. That way, if things crop up during the day, you won't miss your training session," he says. Sure, your boss may ask you to come in early, but usually that request will be made the night before, giving you time to plan a workout later in the day. Not so when you've got your gym bag packed and a last-minute proposal gets tossed on your desk—meaning you're not making it to your date with your weight rack.

One drawback of the morning workout, though, is you may not be fueled properly to give it your best. Lauren Mangainello, M.S., R.D., a New York-based nutritionist and certified personal trainer, points out that you run the risk of running on empty.

“If you tend to skip breakfast, strength training in the morning isn’t your best option,” she says. “Working out on an empty stomach can hinder your progress—if your body doesn’t have enough energy to support your workout, it will start burning through muscle and possibly increase cortisol levels. Plus, you’ll likely be lacking energy and thus not get the most out of your workout.”

With afternoon or evening workouts, though, an empty tank isn’t as likely an option. Plus, studies, like those listed above, have shown increased performance in later workouts. And if you’re putting more into them, well, you’ll probably be reaping more results in return.

The boost in performance likely has to do with the fact our bodies are warmed up and ready to go later in the day, says Brodie.

“[In] strength training we're really training the nervous system, so that makes sense, as the nervous system has had all day to warm-up,” he says.

Now, with evening workouts, you don’t have to worry about sleeping through your alarm. But you do have to consider how it’ll affect your shuteye later on.

If you’re hitting the gym too late in the evening, Mangainello says it could end up working against your gains. “If you’re trying to build muscle, working out too late in the evening can affect our circadian rhythm and make it difficult to fall asleep at night (because of the ‘exercise high’),” she explains. “This can affect muscle development because our bodies need adequate rest in order to build muscle. Working out is important, but muscle growth is actually occurring when we’re resting—so getting 7 to 9 hours of sleep per night is just as crucial.”

Bottom line: The most important considerations for building muscle mass are consistency, nutrition, recovery and ensuring proper training stimulus, says Brodie.

When choosing what time of day to train, the deciding factor should be selecting a time period that allows you to adequately perform in these four key areas. If you’re a night owl whose early-morning wakeup calls leave you zonked—and the thought of putting something in your stomach leaves you queasy—you may want to stick with the evening. But if you’re constantly getting held up at work, or find yourself skipping on the gym to run other errands after leaving the office, you might have to make time for it in the morning.

What's more, Mangainello points out that going against your body’s natural inclination can have a negative impact on your muscle building efforts. “If you’re forcing yourself to workout in the morning when you naturally have more energy in the evening (or vice-versa), this can ‘stress out’ our bodies and increase levels of certain hormones such as cortisol (which can hinder muscle growth),” she explains.

“The key is to be consistent and allow your body to settle into a rhythm,” Brodie says. “Choose what works best for you, stick with it and be consistent.”

And that, Brodie explains, is more important than any small differences seen in studies. “Consistency is what will allow us to see results. It’s one of the main tenets of strength training—no matter what, you need to put consistent pressure on the body in order to see results. If you make it so that you’re consistent in your lifting, you’ll see far better strength gains than missing workouts because you’re trying to cram a workout in at a time that isn’t realistic for you.”