Why am I so exhausted at certain times of the month?

Your energy fluctuates because two hormones take turns each month: oestrogens (mainly estradiol) and progesterone ^[1]. Their back-and-forth directly shapes your stamina, your sleep, and your mood.

One easy way to find your bearings is to picture four inner seasons. After your period, oestrogens rise: energy, motivation, and mental clarity return. Around ovulation, they peak and you often feel at the top of your game ^[1].

After ovulation, progesterone takes over and your body slows down. Then, just before and during your period, both hormones drop: this is often the most draining time of the cycle ^[2].

This pattern varies from woman to woman and cycle to cycle. But it gives you a reference point: once you know where you are, fatigue stops being a worrying mystery.

After ovulation, your body produces large amounts of progesterone. Your brain converts it into allopregnanolone (a molecule that acts on the same receptors as sleeping pills and anti-anxiety medicines) ^[3]. In other words, your body makes its own sedative.

This calming effect explains why you may feel drowsier and less sharp during these two weeks. It is not a lack of willpower; it is chemistry.

Then, two to three days before your period, progesterone and estradiol drop all at once ^[2]. Estradiol had been supporting your serotonin and dopamine, the messengers of stable mood and motivation. Their fall dims everything.

This drop also disrupts sleep: harder to fall asleep, night awakenings, shallower rest ^[2]. About 30% of women report sleep problems before their period ^[3]. Nearly one in two women lives with premenstrual syndrome, and fatigue is one of its common symptoms ^[5].

During your period, fatigue turns more physical. The uterus releases prostaglandins (inflammatory molecules that trigger contractions) that also enter your bloodstream: fatigue, nausea, and headaches can follow ^[1].

Each cycle causes blood loss, and so iron loss. On average, a woman loses 30 to 40 mL per cycle, but some lose more than 80 mL ^[7]. Periods are the leading cause of low iron in women of childbearing age ^[4]. And iron carries oxygen in your blood.

Here is what matters: you can be low on iron without being anaemic. Your haemoglobin may look normal while your stores, measured by ferritin, are at rock bottom. This iron deficiency without anaemia already causes fatigue, trouble concentrating, and hair loss ^[8][9].

The signs to watch for:

• fatigue that does not lift with rest;
• breathlessness during easy efforts, like climbing stairs;
• unusual paleness, hair falling out, a constant feeling of being cold ^[8].

Understanding this is already a big step. From there, you can act at your own pace, with no rigid programme.

• Adapt your pace: place demanding tasks and intense workouts in the first half of your cycle, and gentler activities before your period ^[4].
• Boost iron: red meat is best absorbed; for plant-based iron (lentils, tofu), add vitamin C and keep tea or coffee away from meals ^[10].
• Protect your sleep: a cool room, regular hours, and 30 to 60 extra minutes of rest before your period if your body asks for it ^[3].

If fatigue is intense or persistent, a blood test is simple and useful. Ask for a ferritin test alongside the standard blood count: below 30 micrograms per litre, supplementation may be considered, even with normal haemoglobin ^[7].

To be heard, name exactly what you feel: "I have cyclical fatigue that returns every month and disrupts my daily life. I would like a ferritin test."