The optimal time of day to train is not usually something that people think about. People train when it’s convenient, when they feel like it, when they can. As a result, the decision on when to train is a byproduct of other arrangements. Such a second rate approach to planning your training sessions leads to second rate results.
There is a science to optimizing your training times. By carefully orchestrating your training schedule in accordance with your circadian rhythm, you will be stronger, faster and more powerful. After the training, you will recover better and gain more muscle. You will also increase your flexibility and reduce your chance of injuries.
Sound too good to be true? Read on and in this guide I’ll explain what the best time to work out is based on your personal circadian rhythm.
Circadian rhythm 101
Your circadian (sir-kay-dee-an) rhythm is a daily cycle of biological activity. The biological activity with the most obvious circadian rhythm is your sleep-wake cycle. Think of your body as having an internal clock that regulates when to activate every system. Actually, the part of your brain called the suprachiasmatic nucleus (SCN) has built-in molecular oscillators that function very much like a pacemaker. That’s why the SCN is often called your internal or biological clock. The SCN interacts with virtually every major system in your body, including hormone production and central nervous system activity. Look at the image below for examples of biochemical and physiological events with a 24 hour biorhythm. 
Note that the clock times are relative to your lifestyle and environment, most importantly when you sleep, work and see daylight. Think of the clock times as averages for a regular Joe in the US that works 9 to 5 and sleeps 12 to 8.
The T/C ratio and maximum anabolism
For athletes, systematic daily variations in core body temperature, energy metabolism and hormonal milieu are the most important factors influenced by your circadian rhythm. Let’s first look at testerone, the alpha hormone, and cortisol, the stress hormone.
As you can see in the graphs below, testosterone production is high at night and low during the day .
Cortisol output is low at night, rapidly rises during awakening and then gradually decreases over the course of the day .
The testosterone to cortisol ratio or the T/C ratio is commonly used as a marker of tissue anabolism and a measure of overtraining [1, 7]. As you can extrapolate from the above graphs, the T/C ratio is highest in the afternoon and evening [17, 4, 29]. During this period, exercise causes the smallest increase in cortisol and the largest increase in testosterone [17, 1]. Cortisol elevation also recovers fastest at that time . It is very plausible that the hormonal milieu in the late afternoon is optimal for maximum muscle anabolism. However, the research is still contentious about how transient fluctuations in hormone concentrations relate to muscle growth [4, 6, 29].
Regardless of whether hormones are related or not, muscle hypertrophy signalling is much higher in the afternoon than in the morning according to gene expression and cellular activation research .
Technical note for the hardcore science lovers
The specific factors involved in the higher muscle hypertrophy signalling in the afternoon appear to be the eukaryotic elongation factor protein (eEF2 (Thr56)) and the alpha and beta isoforms of mitogen-activated protein kinase (p38MAPK (Thr180/Tyr182)). The higher activity of eEF2 increases protein synthesis translation elongation capacity. The higher activity of p38MAPK increases cellular growth and differentiation to adapt to the training stress. Akt (Ser473), p70S6 (both Thr389 and Thr421/Ser424), rpS6 (Ser240/244), and Erk1/2 (Thr202/Tyr204) do not seem to be differentially responsive at different times of day.
Core body temperature and maximum performance
Much less contentious are the effects of your core body temperature. Core body temperature is the temperature at which your central organs operate. Enzymatic reactions are extremely sensitive to minor variations in your core body temperature. For the biological systems involved in high intensity physical exercise, the optimal temperature is relatively high. Core body temperature is low at night, rises quickly upon awakening and reaches a maximum in the evening (see graph below ).
Just hot enough to break a world record
The optimal body temperature for strength training normally occurs in the late afternoon to early evening. During this time, you have optimal nerve conduction velocity, joint mobility, glucose metabolism and muscular blood flow. This improves tissue stress distribution, so that your muscles can be maximally activated and your connective tissue remains healthy. 
It is no surprise then that most sports records are broken in the early evening . More importantly, a multitude of randomized, controlled, scientific experiments support exercising in the late afternoon to early evening [12, 13]. At this time, flexibility, power and muscular strength reach their daily peak. Endurance capacity seems to have a less pronounced circadian rhythm [11, 1].
The best time to work out
Based on the circadian rhythm of your hormones, gene expression and your core body temperature, the best time to schedule your training sessions is usually between 14:30 and 20:30 h . If your sessions last longer than an hour, train a bit earlier. This advice is based on the assumption that you generally sleep when it’s dark and are awake when it’s light outside.
For those with an irregular sleep-wake cycle (read: students), it’s preferable to wait at least 6 hours after awakening before training. The optimal training time will then be closer to 20:30 h than 14:30 h.
Tweaking the formula
Individuals differ in the exact timing of their circadian rhythm . The acrophase, or peak performance time, also differs across biological activities. For example, swimming performance peaks a few hours later in the evening than most ground based activities . Peak performance and exercise adaptations correlate strongly, so a good rule is to train when you personally perform best [19, 1]. Realize though that significant individual variation does not imply utter randomness. Starting your day with floor presses from your bed or planking yourself to sleep is not recommended.
When experimenting with at what time your performance is optimal, you can use your heart rate as a guideline. Resting heart rate and core body temperature are strongly correlated . Therefore, the time of day when your resting heart rate peaks is often the best time to train. Oral or insulated axilla (under the armpit) temperature readings are too noisy to measure your circadian rhythm [23, 24]. The scientific gold standard is sticking a thermometer up your ass, but that’s probably reserved for the real die-hards. As some researchers put it, “Core temperature is often measured using a rectal probe, a thermistor inserted 10 to 12 cm [4 to 5 inches] past the anal sphincter. Participants are not always comfortable with this site […]” .
Note for older readers
Peak performance for strength training in middle aged adults occurs earlier than in adolescents. (In the temperature graph above, the black dots represent older subjects.) So, if you’re in your forties or beyond, you should train an hour or two earlier. 
No time for excuses
Not everyone has the luxury of planning their training sessions during the physiologically optimal times. ‘Real life’, whatever it means, has a way of planning things for us. Our schedules have to consider our work, study, family and other day to day activities.
However, for most people these are just excuses. ‘Not having time’ to train actually means ‘I value training less than the thing I’ll do instead’. The things you make time for are the things you prioritize. In Dutch, my native language, the word for priority is pronounced as the Dutch equivalent of prioritime. This makes a lot of linguistic sense to me. Time equals priority.
Most readers of my articles are intrinsically motivated to constantly improve themselves, so I probably don’t have to tell you to prioritize training in your life. So what’s a Bayesian with a genuine scheduling problem to do? You have two options to train in the morning or at night without having your training sessions suffer much as a result.
Nothing like a pre-workout pre-workout
Option one to optimize training in the morning is consuming a caffeine based pre-workout stimulant. A dose of ~250 mg (3 mg/kg) caffeine raises neuromuscular readiness to perform close to afternoon levels .
Note that the researchers of that study concluded that caffeine was sufficient to fully counteract impaired performance in the morning. The AM plus caffeine group still performed worse than the PM group, but this difference was not statistically significant. I suspect this was due to the small sample (N = 12) and resulting insufficient statistical power to detect the performance decrement.
Even aside from the fact that caffeine is not sufficient to boost your physical state to afternoon levels, there are several reasons why taking caffeine in the morning as a pre-workout is not ideal.
• Caffeine decreases the T/C ratio .
• Caffeine doesn’t elevate morning growth hormone levels to afternoon levels .
• If you consume more than 50-100 mg of caffeine daily, you’ll develop a tolerance to caffeine’s ergogenic effects.
Consuming caffeine at night will interfere with your sleep, so either way caffeine use is a temporary solution. All in all, caffeine supplementation is highly beneficial when training in the morning, but it’s still not as optimal as training in the evening (and possibly still using caffeine).
When doing it wrong, consistency is key
The second strategy to increase your performance when training at a suboptimal training time is to always train at that time. Your body will adapt its circadian rhythm to the morning training stress and reduce the performance decrement at that time [5, 20].
However, this adaptation is imperfect for two reasons. The body only adapts if you train at a time that is several hours removed from your physically optimal training time. Otherwise there is no incentive for the body to adapt its circadian rhythm [5, 20].
Secondly, the adaptation is incomplete . The nervous system adapts reasonably well, but physiological systems such as hormone production adapt less well. Several studies have looked at long term muscle size and strength gains in groups training at different times of day. Even when people always train at the same time, strength increases are generally slightly higher and muscle gains up to 84% higher have been found when training in the evening instead of the morning [see graph below; 26, 27, 31].
Unpublished research from professor Scheett at the 2005 NSCA conference came to the same conclusion .
Your body also acclimates to training in the early evening by increasing the circadian variation in performance throughout the day . As such, it is still best to train when your body is biologically primed to do so, because you’ll prime it even further to train at this time. I incorporate meticulous circadian rhythm control in some of my clients and myself and even after supplementation, light therapy and nervous system activity control, we experience better results when consistently training in the early evening instead of in the morning.
The exception to the rule
If you have a job that is particularly stressful, it may be better to train during lunch than after work. Mental stress, such as very high responsibility or long commuting and physical stress, such as manual labor or simply working long hours, takes its toll on the body. The fatigue from work may then offset the benefits of the optimal physical state later in the day. At least one study shows that in shift workers peak performance occurs before work, followed by lunch, followed by after work . So if your work is particularly fatiguing, it may be best to schedule all your training sessions before work or during lunch. Don’t forget to take a pre-workout. Training in the early morning before going to work will take some getting used to.
Not many people think about when the best time to train is. Even fewer have the discipline to schedule their training sessions when they are biologically primed for maximum performance. It’s a shame to put in countless hours in the gym at a time when your body is not in top gear and can’t recover optimally from the training. By following this guide you can optimize your training schedule and fast-track your gains. All without training more or changing your diet.
Take home messages
• Your circadian rhythm is a 24 hour cycle of biological activity set by your internal clock. Your sleep-wake cycle is one of many systems influenced by your circadian rhythm.
• Your hormonal milieu, gene expression and core body temperature have a circadian rhythm. Together, they result in peak physical condition to train and recover in the late afternoon to early evening.
• The best time to work out is generally between 14:30 and 20:30 h if you have a regular sleep-wake cycle.
• You can determine the optimal time to train by monitoring your maximum resting heart rate and training performance.
• If you can’t train when your body is primed to do so, supplement with caffeine pre-workout and schedule your workouts when you can always train.
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