How Much Exercise You Need Per Week Totally Depends on Your Goals

How Many Hours a Week Should You Work Out, Exactly?

If your busy schedule makes it hard to squeeze in a trip to the gym, you may wonder, "how many hours a week should I work out, really?"

Well, even experts argue over that very question — but it turns out, you might not be doing enough (or you may be doing too much). A study published in Circulation found that 30 minutes of exercise a day — the recommended amount — might not be enough to keep your heart healthy.

In fact, you might need four times that. Researchers found that the amount of exercise you get has a direct dose relationship to your heart health — the more you get, the healthier your heart will be — and they suggest two full hours a day of moderate exercise should be the new goal.

But what about a higher-intensity workout such as your favorite HIIT class — don't you get more bang for your buck with that? While you'll certainly burn more calories going harder for short periods of time, moderate exercise does have its value, says Noam Tamir, C.S.C.S., the founder of TS Fitness in New York City. "You will get a nice steady flow of blood flowing into the left ventricle, which needs to be strong because it pumps blood to the rest of the body," he notes. When you get your heart pumping at a slower and steadier pace, it allows the ventricles to fully expand and contract with each beat. "High-intensity training, on the other hand, doesn't do this as effectively, although it is more effective at burning calories," explains Tamir. When you go hard, your heart beats too fast to get the same effect — think of it like the difference between taking short, quick breaths versus long, deep ones.

The bottom line: How many hours a week you should work out will vary depending on your goals. And while every body is different, science does have some stats on how long you should sweat depending on what you're after.

Goal: Have a Healthier Heart

Time at the Gym Per Week: 10 hours

To reap the most heart health benefits, the researchers of the Circulation study are on to something: They found that an exercise regimen of two hours per day reduces the risk of heart disease by 35 percent. And yes, two hours seems like a ton, but study authors were only looking at moderate exercise — so your walk to and from the subway counts! To score more steps, take walking meetings, do a couple laps around the office at lunch, or go for a sunny stroll with your bestie.

That's not to say, though, that the traditional recommendation of 30 minutes per day has no heart-health benefits at all. Rather, this shorter amount of exercise reduces your risk of heart disease by about 10 percent. So, a smaller percentage, but still something.

Tamir recommends his clients seeking a healthier heart regimen do a combination of high intensity and moderate intensity cardio such as HIIT workouts. "This has been shown to significantly improve your body's VO2 max, which is the body's ability to utilize oxygen," he notes. A higher volume of blood flow expands your arteries and minimized the chance of blockages.

Goal: Supercharge Your Strength

Time at the Gym Per Week: 30 minutes

Strength training is about way more than just getting big guns. Studies have shown that regular resistance training reduces your risk of type 2 diabetes and blood pressure and improves your cardiovascular health, not to mention your self-esteem. (Here are 11 other health and fitness benefits of lifting weights.)

The great thing about strength training is that you don't have to devote a lot of time to reaping these benefits. "In many cases, increasing strength can take as little as training once a week," says Tamir. "The difference in gains between training one, two, or three times per week is very minimal," he notes.

Goal: Up Your Endurance

Time at the Gym Per Week: 3 to 4.5 hours

Whether you're in the midst of marathon training or just want to run a 5K without stopping, building endurance is key to a well-rounded workout. To up your endurance, it's all about quality over quantity of gym time. A study published in the Journal of Medicine and Science in Sports and Exercise found that HIIT is more effective than doing the same amount of total work but at a lower intensity. (See also: The Difference Between Muscular Endurance and Muscular Strength, Explained)

But don't bank on settling into just one routine if you want to improve endurance. "The body will adapt and need to be challenged by a more rigorous routine. Otherwise, the benefits associated with the exercise will give a low return," explains Tamir. Try the following endurance plan to keep things fresh, he recommends:

• Day 1: Short run for 20 to 30 minutes; strength train for 20 to 30 minutes
• Day 2: Interval HIIT run for 20 to 30 minutes
• Day 3: Strength train for 40 to 50 minutes
• Day 4: Rest day or brisk walk for 20 to 30 minutes
• Day 5: Hill run for 20 to 30 minutes
• Day 6: Strength train for 40 to 50 minutes
• Day 7: Swim or bike for 30 to 40 minutes

Goal: Be in a Better Mood

Time at the Gym Per Week: 2 to 4 hours

As Elle Woods once famously said, "Exercise gives you endorphins. Endorphins make you happy." (Amen.) Studies have long shown that regular physical activity can help decrease depression and anxiety, and it can even score you a serious natural high with the cocktail of serotonin and dopamine released every time you exercise. (More: The Mental Health Benefits of Exercise, According to Experts)

"Workouts that are as little as 20 minutes per day can create a 12-hour mood boosting benefit," says Tamir. He often sees mood boosts happen within five minutes of starting a training session. For optimal mood boosting benefits, aim for 20 to 40 minutes of moderate exercise a day.

Goal: Get Better Sleep

Time at the Gym Per Week: 2.5 to 3.5 hours

All sleep is not created equal. Your body needs what experts call "slow-wave sleep," which is responsible for body repair and maintenance in order to fully recharge and keep you from feeling sluggish. According to the National Sleep Foundation, exercise is excellent for increasing not just the amount of sleep you get, but the quality.

To get the best zzz's, make time in the morning for a 30- to 45-minute run five times per week. Exercise too vigorously or too close to bedtime, and it could actually have the opposite effect, says Tamir. Why? Since exercise also produces adrenaline, hitting the gym right before you hit the mattress may leave you lying wide awake. (You can do this bedtime yoga routine before bed and still get a good night's sleep, though.)

Goal: Boost Your Brain Power

Time at the Gym Per Week: 2.5 hours

All those squats are doing more than just strengthening your entire lower body — they're also helping you stay sharp. According to researchers at Stanford, exercise immediately boosts your cognitive skills. Researchers have also found that regular exercise, especially between the ages of 25 and 45, decreases your risk of dementia and Alzheimer's. Say hello to a stronger brain! Thirty minutes of high intensity exercise a day should be enough to help you stay sharp. (See also: Squat Therapy Is a Genius Trick for Learning Proper Squat Form)

Goal: Burn More Calories

Time at the Gym Per Week: 40 to 60 minutes

If you're focused on calories burned at the gym, you may think that it's a good idea to toil away in the cardio room for long stretches of time. But luckily, there's a more time-efficient way to reach your goals: Researchers from the University of Wisconsin tested Tabata-style workouts and found that this high-intensity interval training torched 15 calories per minute.

Try a 20-minute Tabata session two to three times a week — giving yourself at least two rest days in between — to reap the most calorie-burning benefits. (Here's the difference between HIIT and Tabata, FYI.)