The Best Exercise Time Is the One You Can Sustain

The next question was submitted by Woody and Woody wants to know when the best time of day to exercise is. It's really an interesting question. You probably know my answer. The best time of day to exercise is when you can consistently do it. If you have a very busy schedule from the first moment you wake up in the morning until 5:00 p.m. When you get off of work and the best time for you to exercise consistently is after work, that is great. Continue to do that.

If you have more of a flexible schedule, then we can talk about the best time of day to optimize this. I want to be clear. When it comes down to it, I don't think it matters. I think the most important thing is just being consistent. Keep that in mind. There have been a few studies that have looked at the timing of exercise in the morning versus later in the afternoon versus the evening. We can discuss those studies, but I don't think timing matters much in the big picture.

Let's start with morning exercise. There are some randomized controlled trials suggesting that if you exercise in the morning you may have greater reductions in body fat compared to doing the exact same exercise later in the day. One of the key reasons for this really comes down to the fact that for whatever reason when you're exercising in the morning, most people are more in a fasted or semifasted state. They don't eat a really big meal before they exercise in the morning. I can absolutely confirm this. I exercise in the morning. That's when I like to exercise.

I exercise in a fasted state. Most of the time I don't have anything to eat before I work out. And that's a major reason why you have increased fat burning because you may already be in ketosis. Depending on how much you eat and other factors, 11 to 12 hours into the night, if you haven't eaten, you start to shift into ketosis. Then when you add exercise on top of that, you're really pushing your body into more of a ketotic state. This increases fat burning. Your body is using fat as a fuel source.

You don't have that extra carbohydrate source around. You're not really converting a lot of glucose into energy. Your body is now using fat that is stored in your adipose tissue through lipolysis, breaking down that fat, bringing it into the bloodstream, then transporting those fatty acids into the mitochondria and burning them as fuel. So that's a major reason why these randomized controlled trials have shown that if you exercise in the morning, you can have greater reductions in body fat. Now that said, you can exercise in a fasted state all you want. If you go and overeat and eat a bunch of potato chips and high caloric foods later in the day, you can erase that deficit and store more fat. Keep that in mind.

There's also a circadian component with morning exercise. So if you exercise in the morning, it tends to shift the body's clock earlier. What I mean is it's going to help with the whole circadian rhythm. It's similar to bright light exposure first thing in the morning. That also shifts the body's clock earlier. It's going to help you make melatonin earlier in the evening. It's going to help you go to sleep earlier.

It's going to help with earlier sleep timing and better sleep quality in some people as well. This can translate to better insulin sensitivity and weight regulation. If you're improving your sleep and circadian rhythm, those effects come along with that whole package. That said, morning exercise is not universally superior for every metabolic outcome. In people with insulin resistance or type 2 diabetes, high-intensity exercise earlier in the morning can sometimes transiently raise blood glucose, while afternoon or early-evening exercise may improve 24-hour glucose control more than morning exercise. Again, it is a transient effect. Ultimately, high-intensity interval training improves glucose control long term whether it is done in the morning or evening. Doing the exercise is more important than the time. Morning exercise may support fat burning and circadian control because it acts as a zeitgeber, a signal to the brain that it is daytime.

It signals your active period and can shift the circadian clock toward an earlier bedtime. This can support melatonin production at a more reasonable hour. That's something to consider. Let's discuss potential advantages of exercising later in the afternoon, perhaps early evening. And these are things I've definitely experienced. Mostly I would say you're going to get a physical performance advantage. So from a circadian biology perspective, late afternoon or early evening, this is when muscles are at their peak function.

Several performance-related variables that really do tend to peak at this time of day. Core body temperature is typically higher. It improves muscle function. Neuromuscular performance is improved. Reaction time improves. All of these will help translate to better strength, better power and endurance capacity. However, I will say a systematic review found no superiority for exercise timing with respect to some other performance-related outcomes like VO2 max.

For VO2 max, it didn't seem to matter whether or not you trained in the early morning versus the later afternoon. The differences appear more relevant to muscular performance, where you may have a better personal record on your squats, for example. And that's something that I have noticed for sure. There's a common concern with people that do exercise in the evening that they're going to disrupt their sleep. I think that the data here is actually a little bit more reassuring. When exercise is performed in the late afternoon or early evening, I would say the majority of studies show there's no negative effect on sleep latency. The studies found no negative effect on the time it takes to fall asleep or on sleep quality.

There doesn't seem to be any effect on those two parameters of sleep. The only time I've seen any sort of problem arise is when you're doing very high-intensity exercise right before bed. For example, a HIIT workout an hour before bed is when you start to see a problem with disrupted sleep. At the end of the day, any exercise is better than none. Make it consistent. Use the routine that works for you. There was a time in my life when my schedule was not as flexible as it is now.

As I mentioned, I exercise in the morning. I love working out in the mornings and that's typically what I do now. However, for many years, when I was a graduate student, when I was a postdoc, when I had to be in the lab early in the morning, I just couldn't get to the gym or workout before I had to be at work. For many years, I went to the gym after work, after I was in the lab. That was fine. It worked out for me. At the end of the day, whatever works best for you, that's my answer.

I'm glad to have gone into some of the details so you have some idea of where this is coming from and why people are interested in exercise timing.