Your hormones change throughout the month. But does your performance? The answer is much less predictable than you might think.
You’re halfway through a run and it feels harder than it should. Your legs feel heavy. Your usual pace suddenly feels less comfortable. Maybe you slept badly. Maybe you’re cramping.
And then you remember:
You’re on your period.
Which raises a pretty reasonable question.
Does that actually matter?
If you’re bleeding, are you losing blood that your body needs to deliver oxygen to your muscles? Could that make you slower? What about iron? And what happens during the rest of the month, when estrogen and progesterone are changing even though you might not feel anything at all?
Maybe you’ve also noticed that one week you feel incredible in the gym, and another week the exact same workout feels strangely difficult.
So you search for an explanation.
And the internet has PLENTY.
Train hard around ovulation.
Avoid intense exercise during your period.
Do endurance work in one phase and strength work in another.
Sync your training to your cycle.
It sounds scientific.
And part of it is.
Your hormones do change throughout the menstrual cycle. They influence systems involved in temperature regulation, metabolism, blood vessels, muscle and the nervous system.
But here’s the part that tends to get lost:
A change in physiology does not automatically mean a change in performance. And that distinction changes almost everything.
Let's break it down! First: what is actually changing?
The menstrual cycle is not simply your period followed by a few weeks of waiting for the next one. Underneath it is a constantly changing hormonal environment.
During the early follicular phase—around the beginning of menstruation—estradiol and progesterone are relatively low.
Estradiol then rises as ovulation approaches.
After ovulation, progesterone increases substantially and estradiol rises again before both eventually fall toward the next period.
That is the simplified version.
Real cycles are WAY messier.
Cycle length varies between women. It can vary within the same woman. Ovulation does not magically happen on Day 14 every month, and hormone concentrations can differ considerably from one cycle to another.
This is actually one of the reasons menstrual-cycle research is so difficult: simply asking someone what “day” of their cycle they are on is not necessarily enough to know what their hormones are doing. Better studies use combinations of cycle tracking, ovulation testing and direct hormone measurements to verify phases. [1–3]
So yes, your physiology is changing.
The more interesting question is whether those changes are large enough to change what happens when you run, lift, cycle or compete.
“But if I’m bleeding, won’t that affect my workout?”
This might be one of the most obvious questions—and one that rarely gets answered properly- the one I am truly always asking myself as well.
Your period does involve blood loss.
But menstruating is not the physiological equivalent of suddenly donating a bag of blood before your run. For most people with typical menstrual bleeding, being on your period does not mean that enough circulating blood suddenly disappears to dramatically reduce oxygen delivery during that morning’s workout.There is, however, a more important connection between menstruation and exercise:
iron. IRON. I.R.O.N
Iron is needed to make hemoglobin, the protein in red blood cells that carries oxygen. Menstrual blood loss contributes to iron loss, and athletes have additional factors that can influence iron balance.
Over time, particularly in someone experiencing heavy menstrual bleeding, inadequate iron intake or high training loads, iron stores can become low. Iron deficiency is common in female athletes and can be associated with impaired aerobic capacity and exercise performance. [4,5]
That means there is an important difference between:
“I am bleeding today, therefore I have less oxygen available.”
AND!
“Repeated menstrual blood loss can contribute to low iron stores over time, which can affect oxygen transport and exercise.”
Those are not the same thing.
If periods are unusually heavy or prolonged, or exercise is accompanied by persistent fatigue, dizziness or an unexplained drop in performance, that deserves more attention than simply blaming the phase of the cycle.
So… does your period make you perform worse?
This is where things get pretty interesting. Because if you look only at the physiology, you could build a very convincing argument that performance should change.
Estradiol may influence carbohydrate and fat metabolism.
Progesterone raises body temperature.
Sex hormones interact with the cardiovascular system.
There are plausible effects on skeletal muscle, the nervous system and recovery.
So researchers expected to find clear changes in performance across the menstrual cycle.
The problem?
They largely haven’t.
A major 2020 systematic review and meta-analysis examined 78 studies and found only a trivial average reduction in exercise performance during the early follicular phase compared with other phases. More importantly, the quality of the evidence was low and there was substantial variability between studies and between women. [1]
Research specifically in elite athletes has reached a similar conclusion: some performance measures have changed in certain studies, but there is no consistent direction or magnitude of effect—and no clearly superior phase for performance. [6]
And when a newer 2025 systematic review included only studies using much stricter hormonal verification, the story still didn’t suddenly become simple. Some outcomes differed across phases, others did not, and the direction of the effects remained inconsistent. [3]
In other words:
the hormones follow a cycle much more reliably than performance does.
What about endurance?
This is probably where the hormone story sounds most convincing.
Estradiol has been linked to changes in fuel metabolism, including greater reliance on fat and potentially greater conservation of carbohydrate stores.
That sounds pretty useful if you’re running a marathon.
But a physiological mechanism is not automatically a measurable performance advantage.
Recent studies in endurance-trained women have done a much better job of actually verifying menstrual phases.
In the FENDURA Project, researchers measured performance-related variables across different menstrual phases in endurance-trained athletes. At the group level, menstrual phase—and circulating estrogen and progesterone concentrations—did not meaningfully alter VO₂peak, running economy, time to exhaustion, lactate-related measures or 30-second power. Individual responses existed, but they were not consistent from one person or cycle to another. [7]
Another study in endurance-trained women compared cycling performance across verified menstrual phases and similarly found no significant difference in time-trial or sprint performance. [8]
A follow-up FENDURA study published in 2025 also found menstrual phase did not significantly alter the ability to maintain running economy following typical low- or high-intensity training sessions. [9]
So could hormones affect metabolism?
Absolutely!
Does that mean everyone becomes a better endurance athlete during a particular phase?
We simply do not have convincing evidence for that.
What about strength?
You may have seen the claim that certain phases are ideal for lifting heavy or building muscle.
Again, there are biologically plausible reasons someone might expect sex hormones to influence neuromuscular function or training adaptation.
But when the research is considered as a whole, there is not strong evidence that maximal strength predictably rises and falls across the menstrual cycle.
That doesn’t mean no individual woman ever feels stronger during one part of her cycle.
SHE MIGHT.
It means we cannot currently take that experience and turn it into a universal rule:
“This is the phase when women should lift heavy.”
Your body is more complicated than a colour-coded calendar.
Why might you feel hotter?
Here is one menstrual-cycle effect that is much clearer.
After ovulation, progesterone rises.
And progesterone is thermogenic.
Core body temperature is generally higher during the luteal phase than during the follicular phase—often by a few tenths of a degree Celsius. [10]
A systematic review looking specifically at exercise in the heat found higher internal body temperature before and after exercise during the luteal phase.
Interestingly, however, this did not translate into significant phase differences in sweat rate, average skin temperature or exercise heart rate in the pooled data. [11]
This is a perfect example of why this topic gets confusing.
Something in your physiology really did change.
Your temperature went up.
But everything downstream did not automatically change with it.
Physiology is connected—but it isn’t always linear. It's actually almost rarely linear!
And then there are symptoms
This may actually be where the conversation becomes most useful.
Because scientific studies often ask:
Does performance, on average, change between menstrual phases?
Your body may be asking a completely different question:
How do I feel today?
Cramps.
Heavy bleeding.
Headaches.
Poor sleep.
Bloating.
Fatigue.
Low mood.
Back pain.
Gastrointestinal symptoms.
These things can absolutely change how a workout feels—even if your VO₂max has not physiologically fallen because you entered a different phase.
In a study of 1,086 athletes across 57 sports, menstrual-related symptoms were extremely common. Among naturally menstruating athletes, 74% reported dysmenorrhea and 78% reported premenstrual symptoms, and athletes frequently perceived these symptoms as affecting aerobic fitness, strength, mental sharpness, balance and sleep. [12]
A systematic review involving more than 6,000 athletes also found menstrual disorders and symptoms to be common, although the studies varied considerably in how they assessed them. [13]
And newer work in 2026 is increasingly pushing the field toward this broader view: instead of treating the menstrual cycle as nothing more than hormone concentrations, researchers are looking at the interaction between symptoms, physical function, psychology, environment and performance. [14]
That makes sense.
Because two women could technically be in the same menstrual phase and have completely different experiences.
One feels nothing.
The other is cramping, slept badly and has a headache.
Calling both workouts simply “early follicular phase” misses a huge part of the physiology that actually matters to the person doing the exercise.
So should you “cycle sync” your training?
This is probably the question everyone wants answered.
Should you build your training program around your menstrual cycle?
Right now, the evidence does NOT support a universal schedule telling all women to train hard in one phase and back off in another.
Even the newest higher-quality reviews continue to find small, inconsistent or highly individual effects. And a 2025 UEFA consensus on menstrual tracking similarly concluded that evidence linking menstrual phases directly to performance remains inconclusive, while tracking may still be useful for identifying symptoms, irregularities and individual patterns. [3,15]
That doesn’t mean tracking your cycle is useless (I have been doing it for years).
Quite the opposite.
It means the purpose of tracking should probably change.
Instead of:
“I’m in my luteal phase, therefore today should be a bad workout.”
Try:
“I’ve noticed that for three cycles in a row, I sleep poorly and my RPE rises two days before my period. Is that a real pattern for me?”
One is a prescription based on what your body is supposed to do.
The other is an observation of what your body actually does.
That distinction really matters.
SEE IT IN YOURSELF
If you’re curious, try tracking your training over three menstrual cycles.
Not because three cycles will turn you into your own randomized controlled trial.
But because patterns become much easier to see when you stop relying on memory.
Record:
Cycle day / bleedingWorkout typePace, power or loadAverage heart rateRPE — how hard it feltSleepSymptomsHow you felt overall
Then look back.
Maybe your performance changes.
Maybe only your perception of effort changes.
Maybe symptoms explain more than phase.
Maybe there is a pattern one month and absolutely none the next.
And maybe you discover that your cycle barely affects your training at all.
That is useful information too.
The goal is not to find the pattern you were told should exist.
It is to see whether one actually exists for you.
THE BOTTOM LINE
Your menstrual cycle is real physiology.
Hormones fluctuate.
Body temperature changes.
Metabolism may shift.
Symptoms can change dramatically.
And menstrual blood loss can contribute to iron deficiency over time.
But none of that means your exercise capacity rises and falls according to a perfectly predictable monthly schedule.
Across studies, objective measures of endurance, strength and performance generally show small, inconsistent or highly individual differences across menstrual phases.
That doesn’t make the menstrual cycle irrelevant.
It makes it personal.
So instead of asking:
“Which phase am I supposed to perform best in?”
Maybe the better question is:
“What actually happens to me?” Track it. Get curious.
Pay attention to your symptoms..and don’t manufacture a limitation your body hasn’t given you.
Your cycle is information—not an instruction manual