High-intensity interval training, or HIIT, is considered by many to be the premier cardiovascular training strategy. One of the main reasons people love HIIT is that it can produce considerable results with a very low training volume. What if you’re not trying to train as little as possible though? How often should you do HIIT? Could you get better results by training more often, perhaps as often as twice a day?
Quite possibly, yes. There are good reasons to consider doing HIIT twice a day—let’s take a look at them.
First Off, Why HIIT?
High-intensity interval training is popular for good reason: The workouts are short and highly effective, allowing you to achieve high levels of cardiovascular fitness in just a few 20- to 30-minute workouts per week.
In studies, HIIT workouts generally produce equal or greater improvements in cardiorespiratory fitness compared to steady-state cardiovascular workouts that are two to three times as long. HIIT has also been proven equally effective in improving insulin sensitivity and blood sugar control in diabetic and pre-diabetic populations, showing potential to not only manage but potentially even reverse type 2 diabetes.
Major cardiorespiratory improvements can be seen with as little as two half-hour sessions of HIIT per week; however, fat loss, improvements in cholesterol levels, and improvements in overall quality of life and subjective well-being require a higher training volume.
In short, HIIT is highly effective at all of the things people want from cardio: fat loss, blood sugar management, and improving one’s overall health. Moreover, it provides those benefits in a very time-efficient manner.
It does have two downsides, though.
First, it’s tough. Although the workouts aren’t long, they make up for that by being extremely intense, and that means you have to push yourself to your very limits. It requires a lot of willpower, motivation, and/or self-discipline to push yourself hard enough to make HIIT work. That means it’s not for everyone, and in practice tends to work better for people who already have a fair amount of training experience.
Second, if your goal is to become a better runner, you need to train in the same style you want to get good at. HIIT will make you better at sprinting, and maybe also at steady-state runs of a similar duration to the whole HIIT workout—that is, a 5K or something around 20-30 minutes in length.
What Are the Benefits of Doing HIIT Twice a Day?
One of the big advantages of high-intensity interval training is that it doesn’t require as high of a training volume, so why wouldn't you wonder how often should you do HIIT? There are a few reasons two-a-days could benefit you.
First, a higher training volume means better results, at least until training volumes get very high, well over ten hours a week for advanced trainees. There are diminishing returns on volume, but they diminish more slowly than most people think.
Second, champion athletes, including runners, typically train twice a day. It clearly works, at least for advanced runners.
Third, training more often allows you to shorten your workouts while still training for more total volume per week. Shorter workouts are easier, which at least ameliorates the main downside of HIIT—the mental difficulty of pushing yourself that hard.
Finally, if you do shorter workouts more often, your workouts will probably be more productive on a minute-for-minute basis. Since you get tired and train less intensely as a workout goes on, the early part of a workout is almost always more productive than the latter part.
Since total training volume is the main driver of physical adaptations to exercise, anything you can do to increase your total training volume—while still being able to recover—is probably good.
So with all that said, twice a day doesn’t mean twice as much total weekly training volume, but it does mean more training volume, and that means more improvement.
The Verdict: How Often Should You Do HIIT?
There's no one-size-fits-all answer to how often should you do HIIT. Twice a day is probably effective, depending on your goals and personality. For most people, the main downside to twice-a-day training is likely to be scheduling. Twice a day tends to work best if you can run outdoors near your home, and not so well if that means driving to the gym twice a day. You’ll need to find two short time slots a day to fit it into your schedule—not necessarily every day, but at least five days a week
Also, some people may actually find motivating themselves to be harder with a twice-per-day schedule. The question to ask here is, what’s harder for you: getting started, or pushing through to the end of your workout?
If you find it harder to finish your workouts as you get fatigued, you’ll find it easier to do shorter workouts more often. On the other hand, if the hardest part for you is getting started, but you find it (relatively) easy to finish once you’ve started, it will probably be easier for you to do longer workouts less frequently.
You can't ask how often should you do HIIT without first considering what other kinds of training you’re doing. Two workouts a day, of any type, should be considered the maximum for almost everyone. That means you don’t want to be doing HIIT fourteen times a week in addition to lifting weights four days a week, and longer runs three days a week.
Finally, consider what you’re training for. HIIT twice a day is great for overall health and amazing for fat loss. In terms of athletic performance, it mainly makes you better at sprinting and short runs. If you’re training for a longer, multi-hour run, like a Spartan race, you’d be well-advised to train the way you’ll be competing—with longer runs.
Of course, nothing stops you from mixing training methods. You could, for example, do six HIIT workouts, four weight sessions, and three longer runs a week. This sort of cross-training is, in fact, how almost all athletes train.