Why Am I So Tired in My 40s? The Real Reasons Women Run on Empty

You sleep. Not brilliantly, maybe — but you sleep. And you still wake up tired. By mid-afternoon you're running on empty. Coffee helps, briefly. But the baseline never seems to come back up.

If you're a woman in your 40s and this sounds familiar, you're not imagining it. And it's not just stress, or a busy life, or needing to exercise more. There are real biological reasons this happens — and most of them are identifiable, and addressable, once you know what you're looking for.

This article walks through the main causes of fatigue in women over 40. Not all of them will apply to you — but if you recognise yourself in several, that's useful information.

First — this isn't normal tiredness

Normal tiredness is proportional. You had a late night; you're tired the next day. You've had a hard week; the weekend restores you. There's a cause, and a recovery.

The fatigue a lot of women describe in their 40s is different. It's pervasive. It doesn't track with sleep. It doesn't improve with rest days. There's a heaviness to it that feels like it comes from somewhere deeper than tiredness.

That distinction matters — because it points toward something biological, not behavioural. Lifestyle changes aren't going to fix a thyroid problem. Getting to bed earlier isn't going to resolve iron deficiency anaemia. Recognising that this is a different kind of tired is the first step to actually addressing it.

The main causes of fatigue in women over 40

1. Perimenopause

Perimenopause can begin in your early-to-mid 40s, and one of its most consistent symptoms is fatigue — driven by several overlapping mechanisms.

Progesterone, which has a sedating, calming effect, begins to decline early in perimenopause. The result is often lighter, less restorative sleep, increased anxiety, and waking at 3am. Even if you're in bed for 8 hours, the quality of sleep you're getting has changed.

Oestrogen fluctuations contribute to night sweats and hot flushes — which further disrupt sleep, sometimes multiple times a night. After months of this, chronic sleep deprivation creates a fatigue that's completely out of proportion to what anyone looking at your calendar would expect.

Perimenopause also affects energy metabolism more directly. Oestrogen plays a role in how your cells use glucose and produce energy in mitochondria. When it fluctuates, energy production becomes less efficient.

2. Thyroid underfunction

An underactive thyroid is one of the most common — and most commonly missed — causes of fatigue in women over 40. The thyroid regulates metabolic rate across every cell in your body. When it slows down, everything slows down.

The complication is that hypothyroid symptoms overlap almost entirely with perimenopause symptoms. And the standard thyroid test (TSH-only) often misses suboptimal thyroid function — particularly when the issue is with T4-to-T3 conversion rather than production. You can feel hypothyroid with a “normal” TSH.

If you haven't had a full thyroid panel (Free T4, Free T3, and TPO antibodies — not just TSH), it's worth requesting one. Read more about thyroid symptoms in women over 40 and exactly what to ask your GP.

3. Iron deficiency and anaemia

Iron deficiency is extremely common in women of reproductive age, and in perimenopause — where periods can become heavier and less predictable — levels can drop significantly. Even iron deficiency without full anaemia (low ferritin, normal haemoglobin) causes fatigue, poor concentration, and breathlessness on exertion.

This is often missed because GPs typically test haemoglobin rather than ferritin. A normal haemoglobin with low ferritin (below about 30–50 ng/mL, depending on symptoms) can absolutely cause the kind of exhaustion you're describing. Ask specifically for a ferritin test.

4. Vitamin B12 deficiency

B12 is essential for red blood cell production, nerve function, and energy metabolism. Deficiency causes fatigue, brain fog, low mood, and tingling in the hands and feet. It's more common than people realise — absorption declines with age, certain medications (particularly metformin and PPIs) deplete it, and hypothyroidism impairs intrinsic factor, which is needed for B12 uptake.

A straightforward blood test picks this up. If your level is below 300 pmol/L (or whatever your lab's reference range lists as optimal rather than just “not deficient”), supplementation or injections can make a significant difference fairly quickly.

5. Vitamin D deficiency

Vitamin D functions more like a hormone than a vitamin — it has receptors on nearly every cell type, including immune cells, muscle cells, and brain cells. Low vitamin D is associated with fatigue, muscle weakness, low mood, and increased susceptibility to illness.

Deficiency is extremely common in northern climates, particularly in winter. Most GPs will test this if asked. Supplementation — typically 1000–4000 IU daily — is safe and often quickly effective.

6. Cortisol dysregulation

Cortisol is your primary stress hormone — it's supposed to be high in the morning (to get you up and going) and low in the evening (so you can sleep). When chronic stress, poor sleep, or perimenopause disrupts this rhythm, you can end up with the opposite: flat cortisol in the morning and elevated cortisol at night. The result is that familiar combination of “wired but tired” — can't sleep, can't wake up properly either.

Cortisol dysregulation isn't something a standard blood test will catch — NHS cortisol tests are usually done to rule out serious pathology (Addison's, Cushing's) rather than detect the milder dysregulation that drives day-to-day fatigue. Addressing it usually comes down to sleep hygiene, stress management, and adaptogenic support.

What to ask your GP for

If you haven't had a recent comprehensive blood panel, now is a good time to ask for one. Specifically:

• Full blood count (FBC) including haemoglobin
• Ferritin (not just iron)
• Full thyroid panel: TSH, Free T4, Free T3, and TPO antibodies — not just TSH
• Vitamin B12 and folate
• Vitamin D (25-OH vitamin D)
• HbA1c (blood sugar regulation — relevant in perimenopause)
• A general metabolic panel (liver, kidney, inflammatory markers)

Write your symptoms down before you go: how long you've been tired, whether sleep helps, any other symptoms you've noticed. The more specific you can be, the more useful the appointment will be.

What you can do now

While you're waiting for tests or appointments, there are nutritional basics that support energy at a cellular level — and that many women in their 40s are running low on:

• Magnesium — involved in hundreds of enzymatic reactions including energy production; commonly depleted by stress. Most people don't get enough from diet alone.
• B vitamins — particularly B12 and B6, both of which support energy metabolism and the nervous system.
• Vitamin D — worth supplementing year-round if you're in a northern climate, regardless of test results.
• Iron — don't supplement without testing; too much iron is harmful. But if your ferritin is low, supplementing (with GP guidance) can be transformative for energy levels.

If your fatigue feels connected to thyroid function or the broader perimenopause picture, the Thyroid & Hormone Health hub is a good starting point for understanding what might be going on — and what support is available.

One last thing

Exhaustion in your 40s is not something you have to accept. It's not just getting older. It's not just being busy. For most women, there's a reason — often several overlapping reasons — and most of them respond to treatment.

Don't let anyone tell you it's normal. Normal is having energy. Push for the investigation.