When do you perform best during training?

When during the day do you actually perform at your best? We explore how the time of day can impact performance and fatigue, focusing on body temperature, hormones, and measurable markers after training. Is there a physiologically “best” time for different types of sessions – like intervals, long runs, and strength workouts? Many seem to perform best in the afternoon, particularly at high intensity, while morning workouts can be suitable for easier sessions and fat focus. However, the practical point is simple: timing can matter, but consistency and looking at the bigger picture over time outweighs perfect timing. Let's dive in! 

Our body isn't like a machine that hits max power the moment we open our eyes. Instead, we go through a physiological startup period before we're ready for tougher physical exertion, especially when the goal is performance. In this article, we highlight how time and differences in the hormonal environment can affect this process. We also discuss blood circulation and the muscles' "ready mode" depending on the time of day you train.

Are there physiologically optimal times for intervals, long runs, strength training, and if so, when exactly are these times?

A review entitled "Effect of Time-of-Day on Muscle Fatigue: A Review" was published in 2013. This review includes 95 studies on the topic, focusing on the impact of time on muscle fatigue—essentially whether muscles tire out more quickly at certain times, with researchers exploring why this time-dependent effect exists and what can be done about it.

In 2017, the same research team released a new review called "Effect of Time-of-Day on Biochemical Markers in Response to Physical Exercise". This review focuses more on parameters we can measure after training and how these vary depending on the time of day you train. It provides insights into how the body adapts to different training stimuli at different times, and the researchers also discuss how training can be scheduled throughout the day to maximize training response.

Both of these studies offer very intriguing information, and we've presented it in a way that doesn't require a PhD to understand. 

Keep this in mind: afternoon sessions from around 5:00 PM to 6:00 PM are usually the best time for most people to perform at a high heart rate. This is true for those who sleep at night and are awake during the day; if you're a night worker, you'll need to reverse the timing. Our peak performance often aligns with our core temperature and various biological markers (see below).

In the afternoon, various biological values are elevated compared to the morning/forenoon:

• URE – Amount of urea (waste product from broken down protein)
• CR – Creatinine (byproduct from muscle metabolism)
• GLC – Glucose
• TG – Triglycerides (fat)
• HDL – High-density lipoproteins

Regardless of when you train during the day, almost all of these markers rise during exercise, resulting in higher levels post-training than pre-training. However, there is a time-dependent difference: all of the above markers tend to be higher after afternoon training compared to the forenoon. This suggests improved fat and glucose metabolism in the afternoon, significantly boosting performance.

A likely explanation is that our catecholamines (stress hormones), such as adrenaline, are elevated in the afternoon. Adrenaline is catabolic (breaking down) as it releases fatty acids from adipose tissue and glucose from the liver, making these energy sources available to the muscles.

Higher creatinine levels also suggest that we are more efficient at resynthesizing ATP from creatine phosphate (an important buffering molecule for ATP). This implies that we can be ready to perform again in the next high-intensity interval, thereby offering a stronger training stimulus in sessions like 4x4-minutes or 4x8-minutes.

Higher postexercise CR levels indicate its robust mobilization (recycled or converted to CR), increased buffering capacity, and phosphocreatine replenishment, which are crucial determinants of the ability to repeat sprints (7).

If afternoons are the most effective time for high-intensity activities like intervals, race pace, and threshold sessions, when should you train for maximal fat adaptation to help your body become better at using fat as fuel?

There's no definitive evidence in these reviews showing that morning workouts are superior for the specific markers studied. However, morning sessions can be practical if your goal is to increase fat oxidation because your glucose and insulin levels are low after a night's fasting. With fewer choices, your body is more inclined to rely on fat as fuel.

According to the 2017 review, you may also have a more efficient antioxidant system in the morning, which helps you better tolerate the increased oxidative stress from fasting workouts. Researchers aren't entirely sure why our antioxidant system is heightened in the morning (measured at 06:00), but they speculate that melatonin, which is higher in the morning, could have a strong antioxidant effect.

Inflammation is often beneficial. We always experience a certain degree of inflammation when we train, and it's an early step to creating a response in the body: handling training stress, repairing damage, and getting stronger for future challenges. However, at certain stages, we may not want too much inflammation, for example, during competition season when quick recovery is more important than maximum training signaling.

We've previously discussed this in our article on antioxidants, and these reviews contain an entire section about C-reactive protein. This protein is measured, for example, in healthcare (CRP test) to provide an indication of the degree of infection or inflammation.

Some studies show no circadian rhythm in C-reactive protein. But a couple of studies that examined how C-reactive protein is affected by training at different times have found that training around 3-5 PM, that is in the afternoon, leads to the highest inflammation. Another study that examined inflammation at three times (08.00, 14.00, 18.00) after a strength workout saw no difference between 2 PM and 6 PM, but inflammation in the form of increased levels of C-reactive protein was higher after training at both 2 PM and 6 PM compared to morning training at 8 AM.

Broadly speaking, this aligns well with earlier sections suggesting that the body is more antioxidant-awesome in the morning. Our hypothesis is that the body recovers faster from morning training compared to the afternoon. Maybe the response and training effect is somewhat lower in the morning, largely because we humans tend to perform poorest in the morning when intensity is high. But if you want to do circulation sessions or throw in a few 4x4-minute intervals during competition season, it might be wise to sometimes do the session in the morning and then eat plenty of carbohydrate-rich and protein-rich food, along with cherries after the session.

Our body temperature follows a circadian rhythm: we're coolest around 6:00 a.m. and warmest around 6:00 p.m. in the afternoon. After that, the temperature drops again in preparation for the evening. As previously mentioned, body temperature syncs well with performance, so once again, everything points to around 6 p.m. as a good time for intervals – yes, even if you need to come up with excuses for not training earlier. 

The temperature varies by about 0.5-1 degree throughout the day (average 0.8 degrees), and the difference exists both at rest and before and after training. If we measure the body temperature of people doing the same session twice (morning and evening), it's higher after the evening session even if all other parameters are the same.

It might sound contradictory that a warmer body can perform better; we often associate heat with overheating and often perform better in a cool climate. But an increased core temperature boosts the activity of many enzymes, including phosphofructokinase and lactate dehydrogenase, both important in substrate metabolism. This makes more glucose and lactate available, quick and efficient fuels for the muscles, and blood flow is somewhat better in the afternoon.

Joch and Uckert. (36) showed that an increase of 18 C in body temperature generates an increase in nerve conduction velocity by 2.4 m/s, and an increase of 28 C provides greater efficiency in body biochemical reactions. At the muscle level, Mastérovoi (43) showed that this temperature increase enhances the vascularization of muscle groups. These temperature changes affect physical performance.

We keep it simple and sum up the entire article in three main points.

• High-intensity and heart rate training in the afternoon, ideally between 17:00 and 19:00 if possible
• Fat adaptation and low-intensity training in the morning
• During the competition season, it might be wise to also include VO2max training like 4x4-minute intervals and similar in the morning

Listen to your body, experiment with your own rhythm — and if all else fails: train when it fits the schedule. You'll still learn something, and sometimes the best tip is not to be too dogmatic.