Training, Diet, or Sleep: Which Is Most Important for Fitness and Performance?

In last week’s post, I addressed a fundamental question: What are the major objectives of an endurance athlete’s diet? In this post I would like to tackle an even more basic question, which I’ve already given away in the title. Namely: Which is most important for endurance fitness and performance—training, diet, or sleep?

As you’re about to see, there’s no simple answer to this question. But attempting to answer it is nevertheless a worthwhile exercise, because it yields clarity on the role of each of these three factors in relation to your athletic ambitions.

The All-or-Nothing Angle

Sleep is a mysterious phenomenon that has long eluded scientists’ efforts to fully explain it. As neuroscientist Michael Halassa confessed in a 2017 article published on, “It’s sort of embarrassing. It’s obvious why we need to eat, for example, and reproduce . . . but it’s not clear why we need to sleep at all.” What isclear is that we literally can’t live without sleep. The longest any human has been known to survive without sleep is just 11 days.

Arguably, this makes sleep even more important than food. The average person can go about 40 days without eating before succumbing to starvation. 

As for training (i.e., exercise), it is, of course, not required for survival, though a case can be made that some amount of physical activity is needed to achieve a normal lifespan, as people who are unable to move their bodies (i.e., sufferers of paralysis) don’t live as long as people who are.

In light of these facts, we can say definitively that if you were going to attempt to complete an endurance race either without training, without eating, or without sleeping, your best move would be to skip the training in favor of eating and sleeping.

The Realistic Angle

Thankfully, you will never have to make the choice I just presented. We live in a relatively stable society in which most people have plenty of food to eat and a comfortable bed to sleep in. So, let’s now approach the question of whether training, diet, or sleep is most important for endurance fitness and performance from a more realistic angle. 

Although I just got through saying that in our society most people have a comfortable bed to sleep in, the modern lifestyle is such that a large fraction of us do not spend enough time in bed and do not get enough sleep. Research suggests that the kind of chronic, mild sleep deprivation that is so common in our society has a bad effect on endurance performance. A 2016 study by researchers at UC San Francisco, for example, found that cyclists whose sleep was restricted to four hours per night for three nights experienced a 2.9 percent decrease in maximal aerobic power and a 10.7 percent decrease in time to exhaustion at VO2max. True, few athletes get only four hours of sleep per night as a matter of habit, but it’s reasonable to assume that longer periods of milder sleep deprivation probably have a similar effect.

Similarly, although most athletes get enough to eat overall, a majority of athletes also fall well short of eating optimally to support their fitness and performance. Common mistakes include poor diet quality, overeating, and within-day energy deficiencies, all of which are proven to negatively affect endurance fitness and performance.

And then there’s training. What’s different about training, from the realistic perspective, is that, whereas everyone sleeps and eats, only a minority of adults in our society exercise regularly. This makes the transition from sedentariness to endurance training a rather common phenomenon. Thus, in the case of training, the realistic scenario isn’t all that different from the all-or-nothing scenario.

There’s plenty of research on how the transition from sedentariness to endurance training affects endurance performance. One example is a 2019 study by Spanish and German researchers, which found that 12 weeks of endurance training increased VO2max by 11 percent and time to exhaustion by 14 percent in a group of previously sedentary adults. Those are big numbers. And it should be noted that sedentary individuals can’t exactly leap straight into heavy training workloads right off the couch. The subjects in this study completed just three low-intensity sessions per week totaling 2.5 hours. Given what we know about the dose-response relationship between endurance training and fitness and performance, it’s safe to say that these folks would have experienced vastly greater improvements over time if they had continued to train in a progressive manner.

Indeed, studies investigating the effects of different training programs in already-fit athletes show tremendous potential for improvement in going from imperfect training to optimized training. A 2014 study conducted at Salzburg Universityreported improvements ranging from 6.2 percent and 17.4 percent in time to exhaustion among experienced endurance athletes placed on one of four different training programs for nine weeks.

Comparing the above-referenced data on sleep, diet, and training leads us to the conclusion that, in the realistic scenario, training offers far greater potential for improvement in endurance fitness and performance than does either sleep or diet. In other words, if you are a typical athlete who doesn’t get quite enough sleep, has a mediocre diet, and trains less than optimally, and you can only change one of these things, your best move is to optimize your training.

The Bottom Line

So, which is most important: diet, sleep, or training? The answer, we now see, is that training, on the one hand, and diet and sleep, on the other hand, are important in different ways. Most athletes place greater emphasis on training, and they are right to do so in the sense that, realistically, getting the training piece right will have a greater impact than getting either the diet or the sleep piece right.

However, as we saw in exploring the all-or-nothing angle, diet and sleep are more foundational than training. Fitness is really just an extension of health, and diet and sleep are more important to basic health than training is. Therefore, any athlete who wishes to get the most out of optimized training should make every effort to get the diet and sleep pieces right as well.