Fatigue that sleep doesn't fix

"Tired all the time" is one of the most common reasons people see a doctor and one of the least satisfying to present, because it is non-specific enough to be filed under stress and sent home.

It is worth working through methodically, because most of the causes are testable and several are easy to treat.

Start by checking it really is enough sleep

Duration is a poor proxy. Fragmented sleep impairs function more than shorter continuous sleep of the same total length, so eight hours interrupted fifteen times is not eight hours.

The two big under-diagnosed causes of fragmentation in women:

Obstructive sleep apnoea, which presents differently in women — insomnia, fatigue, morning headache, low mood, rather than the classic loud snoring and daytime sleepiness. If you sleep long enough and never feel restored, ask specifically for a sleep study. Why women sleep worse covers why it gets missed.

Restless legs, strongly associated with low ferritin and often not mentioned to a doctor because people do not think of it as a medical symptom.

Iron

The most common deficiency in menstruating women and the most commonly missed, because a full blood count can be entirely normal while iron stores are empty.

Ask for ferritin specifically. And ask for the number — many labs flag anything above 12 or 15 µg/L as normal, while symptoms are common well above that. Low ferritin and low mood covers the thresholds and how to replace it properly.

If your periods are heavy, the deficiency will recur until the bleeding is addressed. How heavy is too heavy.

Thyroid

Hypothyroidism produces fatigue, cold intolerance, weight gain, constipation, dry skin, hair thinning and low mood. It is far more common in women and more common with age.

A TSH is the screening test. If it is borderline, free T4 and thyroid antibodies are reasonable to ask for.

B12, folate and vitamin D

B12 deficiency is common in vegetarians and vegans, in people on long-term metformin or proton pump inhibitors, and in older adults. It causes fatigue and cognitive symptoms and, untreated, neurological damage.

Vitamin D deficiency is extremely common at higher latitudes. The evidence that supplementing improves fatigue in people who are not deficient is weak; in people who are, correcting it is reasonable.

Perimenopause

Fatigue is one of the most reported symptoms of the transition, driven by disrupted sleep, night sweats and hormonal fluctuation. It frequently arrives before any noticeable change in periods, which is why it is not connected. Perimenopause can start in your late thirties.

Depression and anxiety

Fatigue is a core symptom of depression rather than a side effect. And anxiety is exhausting in a literal, physiological way — sustained arousal consumes energy.

The direction of causation is genuinely hard to untangle here, and it does not need to be untangled before treating. How to tell a bad week from something that needs help covers where the threshold sits.

Coeliac disease

Substantially under-diagnosed, and a large proportion of people with it present without obvious gut symptoms — fatigue and iron deficiency can be the only signs. Serology is a simple blood test, and it must be done while still eating gluten.

The rest of the list

Diabetes and pre-diabetes. Chronic kidney or liver disease. Heart failure, which presents atypically in women. Sleep-disrupting medications, including some antidepressants and beta blockers. Antihistamines. Chronic infection. Post-viral fatigue, including long COVID, which needs to be named explicitly if the onset followed an illness. Myalgic encephalomyelitis / chronic fatigue syndrome, where post-exertional malaise — symptoms worsening 12 to 48 hours after activity — is the distinguishing feature and where the wrong advice does harm.

And the one that is neither a deficiency nor a disease: chronic under-recovery from a combination of work, caregiving and training with no slack in it. This is real, common, and not diagnosable by blood test.

The panel worth asking for

Full blood count, ferritin, TSH, B12, folate, vitamin D, HbA1c, and coeliac serology. Add liver and kidney function and a calcium if the picture is unclear.

That set catches the large majority of treatable medical causes and costs very little. If you are told your bloods are normal, ask which tests were run — "normal bloods" frequently means a full blood count and nothing else. How to prepare for a ten-minute appointment has the specific phrasing.

The pattern is the most useful clue you have

Fatigue that is worse in the second half of your cycle points somewhere different from fatigue that is worse after poor sleep, which points somewhere different from fatigue that is worse two days after exercise, which points somewhere different from fatigue that is constant.

Almost nobody can answer that question from memory. The recall problem is severe here specifically because fatigue distorts how you remember the last few weeks.

A daily energy rating, logged against sleep, cycle day, activity and mood, answers it in about six weeks — and it usually narrows the list substantially before you see anyone. Naked is built for exactly that: not one more thing to fill in, but a record that turns "tired all the time" into "tired on these days, following these things", which is a question a doctor can actually work with.