Why You Feel Tired All the Time — The Science of Fatigue

Feeling tired all the time is one of the most common complaints in primary care — yet it is often dismissed as stress or lifestyle. The reality is that persistent fatigue almost always has a physiological driver. And in women, several causes are systematically underdiagnosed because fatigue is frequently attributed to emotional or behavioural factors rather than investigated biochemically.

This is a science-based breakdown of the most common, evidence-supported causes of chronic tiredness — and what to actually do about each one.

1. Iron Deficiency (With or Without Anaemia)

Iron deficiency is the most common nutritional deficiency in the world, and menstruating women are disproportionately affected. Here is the critical distinction that is often missed in clinical practice: you do not need to be anaemic to experience significant fatigue from iron deficiency.

Iron is required for haemoglobin synthesis (oxygen transport) — but it is also a cofactor in mitochondrial energy production, dopamine synthesis and thyroid hormone metabolism. Low ferritin (iron stores) causes fatigue even when haemoglobin is within the normal range. Many women are told their blood count is "fine" when their ferritin is at 12 µg/L — technically in range, but clinically low enough to cause significant symptoms.

Research suggests optimal ferritin for symptom-free function is above 50–70 µg/L, not simply above the laboratory minimum of 12–15. If you are consistently tired and menstruating, ask specifically for a serum ferritin test — not just a full blood count. Eating iron-rich foods alongside vitamin C and avoiding tea and coffee within an hour of meals maximises iron absorption. See our post on nutrition for each cycle phase for phase-specific iron strategies.

2. Thyroid Dysfunction

The thyroid gland produces hormones (T3 and T4) that regulate the metabolic rate of virtually every cell in the body. In hypothyroidism (underactive thyroid), metabolic rate slows systemically — affecting mitochondrial activity, body temperature regulation, heart rate, cognitive speed and — prominently — energy levels.

Hypothyroidism is significantly more common in women than men and is frequently under-detected because symptoms develop gradually and can be attributed to lifestyle factors. Fatigue in hypothyroidism is often described as profound, heavy tiredness — not just sleepiness. It is frequently accompanied by cold intolerance, weight changes, hair loss, constipation and brain fog.

Hashimoto's thyroiditis — an autoimmune form of hypothyroidism — is the most common cause of hypothyroidism in developed countries. Diagnosis requires a TSH blood test and ideally free T3 and T4 levels. The NHS guidance on hypothyroidism recommends testing if you have persistent, unexplained fatigue alongside other symptoms.

3. Cortisol Dysregulation and Chronic Stress

Cortisol is your primary stress hormone. Under acute stress, cortisol is adaptive — it mobilises energy, sharpens focus and prepares you for action. Under chronic, low-grade stress (the modern default), the cortisol response system can become dysregulated.

Chronically elevated cortisol disrupts sleep architecture — specifically reducing the amount of restorative slow-wave sleep and REM sleep. It also promotes catabolism (muscle breakdown), increases blood sugar fluctuation, suppresses the immune system and eventually leads to what is sometimes clinically described as HPA (hypothalamic-pituitary-adrenal) axis dysregulation — where the feedback loop that normally turns off the cortisol response becomes blunted.

The result is a paradox: you feel wired but exhausted — unable to fully relax or fully recover. Addressing cortisol dysregulation is less about supplements and more about consistent sleep timing, evidence-based stress management, reducing chronic stressors where possible, and physical activity (which normalises the HPA axis). See our science-backed daily routine for practical habits with evidence behind them.

4. Poor Sleep Quality (Not Just Quantity)

Eight hours of fragmented or shallow sleep does not equal eight hours of restorative sleep. Adenosine — the sleep pressure molecule that accumulates during waking hours — must be cleared during sleep. If sleep architecture is disrupted (insufficient slow-wave sleep or REM), adenosine clearance is incomplete and you wake still carrying sleep debt.

Common disruptors of sleep quality: inconsistent sleep-wake timing (irregular circadian rhythm), alcohol (which fragments sleep and suppresses REM), blue light exposure within 60–90 minutes of bedtime (delays melatonin onset), caffeine consumed after midday (half-life of 5–7 hours means afternoon coffee still affects midnight sleep), and room temperature above 18°C (core body temperature must drop to initiate and maintain deep sleep).

5. Vitamin D Deficiency

Vitamin D receptors are present in virtually every tissue in the body, including muscle cells and immune cells. Low vitamin D is associated with fatigue, muscle weakness, low mood and increased infection susceptibility. In populations with limited sun exposure — which includes the majority of people living above 35° latitude from October to March — deficiency is extremely common.

Vitamin D deficiency is diagnosed via serum 25-hydroxyvitamin D blood test. Supplementation is generally recommended at 1000–2000 IU daily for maintenance in deficient populations — though corrective doses are higher and ideally guided by a GP based on blood levels.

6. Vitamin B12 Deficiency

Vitamin B12 is essential for red blood cell formation, neurological function and DNA synthesis. Deficiency causes macrocytic anaemia (large, poorly functioning red cells), peripheral neuropathy and — notably — severe, sometimes disabling fatigue. Those at highest risk: vegans and vegetarians (B12 is found almost exclusively in animal products), people with conditions affecting absorption (pernicious anaemia, Crohn's disease, coeliac disease) and those on long-term metformin or proton pump inhibitors.

7. Blood Sugar Dysregulation

Meals high in refined carbohydrates and low in protein, fat and fibre cause rapid blood glucose spikes followed by sharp drops. These drops trigger symptoms of hypoglycaemia — fatigue, difficulty concentrating, irritability and hunger. This cycle, repeated across multiple meals per day, creates a pattern of energy fluctuation that many people normalise as "just how they feel." Eating balanced meals with protein, fat and fibre at every meal stabilises blood glucose and sustains energy more consistently.

When to See a Doctor

If you have been persistently tired for more than four weeks without a clear lifestyle explanation, a basic blood panel is warranted. Ask for: full blood count, serum ferritin, thyroid function (TSH, free T3, free T4), vitamin D (25-OH), vitamin B12, fasting glucose, and inflammatory markers (CRP, ESR). These cover the most common biological causes and are standard in most healthcare systems.