Circadian cycles, nutrition, and strength training: How to use your biological clock to perform and grow more by Roberto Castellano

For years, sports nutrition has been discussed as if the body were a flat machine, without hours, rhythms, or context. You eat protein, train, rest, and that’s it. The problem is that the human body doesn’t work that way. It works with rhythms, biological windows, and an internal clock that decides when you are strongest, when you assimilate nutrients best, when you recover faster, and when you are simply wasting your time trying to perform as if it were five in the afternoon when it is actually six in the morning. This makes sense as soon as we understand that humans evolved in a world with specific characteristics, the most significant being the difference between day and night.

That internal clock is governed by circadian cycles, and understanding them completely changes how you should approach both nutrition and strength training. Not for its own sake, but so you can stop fighting your physiology and start using it to your advantage.

WHAT CIRCADIAN CYCLES ARE AND WHY THEY AFFECT YOU, EVEN IF YOU IGNORE THEM

Circadian cycles are biological rhythms of approximately 24 hours that regulate key bodily functions such as hormone secretion, body temperature, alertness, nutrient metabolism, insulin sensitivity, and repair and growth processes. They are governed mainly by the suprachiasmatic nucleus of the hypothalamus, which acts as a synchronized central clock, primarily regulated by sunlight, although other factors such as food, exercise, and sleep schedules also play a role.

Translated into gym language: you are not the same organism in the morning as you are in the afternoon, even if you weigh the same and eat the same. Your ability to generate force, tolerate volume, use carbohydrates, oxidize fat, or activate protein synthesis changes throughout the day. Ignoring this does not prevent you from training, but it does prevent you from optimizing.

The problem is that most people live in a constant state of circadian misalignment: they sleep late, eat at irregular times, get little natural light, train whenever they can, and then wonder why their performance is inconsistent or why their recovery does not keep up.

CIRCADIAN CYCLES AND KEY HORMONES FOR STRENGTH AND HYPERTROPHY

When it comes to strength training, several hormones and physiological variables follow a clear circadian pattern and directly influence performance and hypertrophy.

Cortisol, for example, reaches its natural peak in the morning, shortly after waking. This is not a bad thing; in fact, it is necessary to activate us, mobilize energy, and get us moving. The problem arises when cortisol remains elevated all day because of stress, poor sleep management, or poorly timed workouts.

Testosterone, on the other hand, is usually higher in the morning, but its effective use for performance does not depend solely on its peak; it also depends on the metabolic and neuromuscular context. Having high testosterone at 8:00 does not mean you are stronger at that time.

Body temperature, one of the best predictors of strength and power performance, generally reaches its highest point in the afternoon, between 4:00 and 8:00 p.m. The higher your body temperature, the better your nerve transmission, the greater your muscle elasticity, and the lower your risk of injury. You only have to compare training in the cold with training in a warm environment; the difference in performance is substantial.

Insulin sensitivity also varies throughout the day, generally being better during the first half of the day and gradually worsening at night, although this depends greatly on each person’s activity level and metabolic state, as well as on training. If you train in the afternoon, you will be more sensitive during the later hours of the day, after training.

All of this leads to an inconvenient conclusion for lovers of universal routines: there is no perfect training time for everyone, but there are times that are more consistent with human physiology for specific goals.

STRENGTH TRAINING IN THE AFTERNOON: WHEN THE BODY IS “DESIGNED” TO PERFORM

When it comes purely to strength, power, and work capacity, the afternoon is usually the most favorable time for most people. Body temperature is elevated, the nervous system is more alert, joints are better lubricated, and perceived exertion is generally lower.

From a nutritional standpoint, training in the afternoon allows for something very useful: arriving at the workout having eaten several meals, with fuller muscle glycogen stores and a hormonal environment more favorable for tolerating volume. This is especially useful during hypertrophy phases, when training volume is high and recovery is essential. If your goal were fat burning or metabolic efficiency, things would be different, but we cannot cover every possibility here.

In the context of training around midday, a nutritional strategy consistent with circadian cycles would be:

– Breakfast and early meals with sufficient energy—not excessive, but without absurd restrictions.

– A pre-workout meal that is easy to digest, complete, and not too large, so it does not interfere with training.

– A post-workout meal focused on recovery, with carbohydrates to replenish glycogen and protein to stimulate protein synthesis.

This follows physiological logic. You eat when you are going to use the nutrients most, train when you perform best, and recover when your body is prepared to do so.

STRENGTH TRAINING IN THE MORNING: LESS IDEAL, BUT NOT USELESS

That said, not everyone can train in the afternoon. This is where the second approach comes in—one that works better for others because of their habits and lifestyles, and is very useful when understood properly rather than treated as dogma.

In the morning, catecholamines—adrenaline and noradrenaline—rise naturally, increasing alertness, focus, and fat mobilization. This explains why many people feel mentally sharper early in the day, even if they are physically somewhat stiffer.

Strength training in the morning, especially early, has clear drawbacks: lower body temperature, greater muscle stiffness, and, in many cases, lower peak performance. But it can also have advantages if nutrition and expectations are adjusted properly.

Training fasted or with minimal intake can be useful in very specific contexts, not so much for direct hypertrophy, but for improving adherence, taking advantage of mental focus, and structuring the day. That said, if hypertrophy is the main goal, these workouts should be shorter, involve less volume, and be better planned, while post-workout nutrition becomes even more important.

The key here is not the fast itself, but what you do afterward. A well-planned post-workout breakfast or meal, rich in protein and carbohydrates, can compensate for much of the less favorable context of morning training.

CHRONONUTRITION: HOW TO EAT WITHOUT FIGHTING YOUR BIOLOGICAL CLOCK

Chrononutrition is not about eating at “magical times,” but about respecting the body’s natural rhythms and avoiding contradictory signals.

Eating large amounts very late, for example, can interfere with melatonin secretion and worsen sleep quality, which in the medium term negatively affects recovery, testosterone, and body composition. This does not mean you cannot eat carbohydrates at dinner; it means that chronic, late-night excess has consequences, especially when stomach fullness disrupts nighttime rest.

On the other hand, concentrating most of your calories around the active period of the day, especially near your workout, is generally more efficient for both performance and recovery.

A reasonable strategy for someone who strength trains could be:

– A greater calorie and carbohydrate intake during the first half of the day and around training.

– Lighter dinners with enough protein and no unnecessary excess calories.

– Relatively consistent meal times to avoid disrupting peripheral rhythms.

CIRCADIAN CYCLES, SLEEP, AND RECOVERY: THE PART EVERYONE KNOWS BUT FEW APPLY

You can get your nutrition and training exactly right, but if you sleep poorly, everything above loses its effectiveness. Sleep is when recovery is consolidated, growth hormone is released, and tissues are repaired. Sleep, in turn, depends on how you have managed light, food, and activity throughout the day.

Training very late, eating excessively late, or exposing yourself to screens until dawn—even if that is “modern life”—is a form of sabotage to your performance that, unfortunately, has become normalized. In the medium term, this translates into worse performance, poorer body composition, and stagnation.

From a circadian perspective, consistency beats perfection. Sleeping and eating at similar times each day has a greater impact than seeking the perfect protocol once a week.

PRACTICAL APPLICATION FOR HYPERTROPHY

If your goal is to gain muscle mass, the final message is clear: adapt your training and nutrition to your biology, not the other way around.

If you can train in the afternoon, take advantage of that performance peak and pair it with nutrition rich in carbohydrates and protein before and after training.

If you train in the morning, adjust the volume, focus on a proper warm-up, and prioritize good post-workout nutrition.

In both cases, respect your sleep, maintain consistent schedules, and avoid constantly sending contradictory signals.

Hypertrophy does not depend only on how much you train or eat, but also on when you do it and in what physiological context.