SLEEP & RECOVERY
Sleep Blood Test UK: The 8 Biomarkers That Affect How Well You Sleep
One in three UK adults sleeps poorly, according to the NHS. Most people blame screens, caffeine or stress — and those matter. But a growing body of research shows that nutrient deficiencies, thyroid dysfunction, hormonal imbalances and chronic inflammation can quietly wreck sleep quality even when your sleep hygiene is excellent.
This guide covers the eight blood biomarkers most strongly linked to insomnia, night waking and non-restorative sleep. For each, we explain the mechanism, the evidence, the NHS reference range versus the optimal range for sleep quality, and what to do if your results come back low.
Why Blood Tests Matter for Sleep
Sleep is not just a behavioural issue. Your body needs raw materials to produce melatonin, regulate cortisol, maintain core temperature, sustain slow-wave sleep and keep your airways open. When those raw materials are depleted — iron, B-vitamins, vitamin D, magnesium — sleep architecture degrades even if you are doing everything else right.
The NICE Clinical Knowledge Summary on insomnia recommends investigating secondary causes when sleep problems persist beyond four weeks. Secondary causes include thyroid disease, iron-deficiency anaemia, chronic pain and depression — all of which are detectable on a blood test.
A 2023 systematic review in Sleep Medicine Reviews found that vitamin D deficiency, iron deficiency and magnesium depletion were independently associated with shorter sleep duration, increased sleep latency and poorer self-reported sleep quality across 31 observational studies.
The logic is straightforward: if your persistent tiredness coexists with broken sleep, a standard “sleep hygiene” checklist is treating symptoms, not causes. A blood test treats causes.
1. Ferritin — Your Iron Stores
Ferritin is your body's iron storage protein. Low ferritin is the most common nutritional deficiency worldwide and one of the most treatable causes of poor sleep.
Iron is essential for dopamine synthesis in the brain. Dopamine is the precursor to the neurological pathways that suppress restless-leg sensations during sleep. A landmark Sleep Medicine Reviews meta-analysis (2013) confirmed that serum ferritin below 75 µg/L is associated with a significantly increased risk of restless legs syndrome (RLS), which affects up to 10% of UK adults.
The NHS defines ferritin deficiency at below 30 µg/L, but the NICE guideline for iron deficiency acknowledges that symptoms can appear well above that threshold. Sleep clinicians increasingly use 75 µg/L as the target for optimal sleep, particularly in patients with RLS.
Who is at risk: menstruating women, vegetarians, endurance athletes, blood donors, people taking proton pump inhibitors. If your ferritin is below 50, it is worth investigating as a contributor to poor sleep — see our iron deficiency guide for more detail.
2. Thyroid TSH
Your thyroid sets the metabolic rate of every cell in your body, including the neurons that regulate your sleep-wake cycle. Both hypothyroidism (underactive) and hyperthyroidism (overactive) disrupt sleep — but in different ways.
Hypothyroidism causes excessive daytime sleepiness combined with poor-quality night sleep. A 2019 study in the Journal of Clinical Endocrinology & Metabolism found that patients with subclinical hypothyroidism (TSH between 4.5 and 10 mIU/L) reported significantly worse sleep quality on the Pittsburgh Sleep Quality Index compared to euthyroid controls.
Hyperthyroidism produces the opposite pattern: difficulty falling asleep, racing heart at night, and night sweats. The NHS lists insomnia as a primary symptom of an overactive thyroid.
A full thyroid panel — TSH plus free T4 and free T3 — is far more revealing than TSH alone. See our thyroid blood test guide for the full picture.
3. Vitamin D
Vitamin D receptors are expressed throughout the brain regions that regulate sleep, including the hypothalamus and brainstem. The mechanism is not fully understood, but the epidemiological signal is clear.
A 2018 meta-analysis of 9 studies published in Nutrients found that vitamin D deficiency (below 50 nmol/L) was associated with a 1.5-fold increased risk of poor sleep quality and shorter sleep duration. A larger 2022 study in Sleep Medicine confirmed the relationship: each 25 nmol/L increase in serum 25(OH)D was associated with 7% lower odds of sleep disturbance.
In the UK, the National Diet and Nutrition Survey (NDNS) shows that roughly 1 in 5 adults has a serum vitamin D below 25 nmol/L (frank deficiency), rising to nearly 40% in winter months. The NHS threshold for “adequate” is 25 nmol/L, but most sleep researchers consider 75–125 nmol/L the optimal window.
See our vitamin D deficiency guide for dosing, supplementation timing and the evidence on vitamin D3 versus D2.
4. Magnesium
Magnesium is a cofactor in over 300 enzymatic reactions, including the synthesis of melatonin and the activation of GABA receptors — the brain's primary “calm down” neurotransmitter. Without adequate magnesium, your nervous system stays in a state of heightened excitability that makes it harder to fall asleep and easier to wake.
A 2012 randomised controlled trial in the Journal of Research in Medical Sciences gave 500 mg magnesium or placebo to 46 elderly adults with insomnia for 8 weeks. The magnesium group had significantly improved sleep time, sleep efficiency and serum melatonin levels, plus reduced serum cortisol.
A more recent 2023 systematic review in BMC Complementary Medicine and Therapies concluded that magnesium supplementation was associated with improved subjective sleep quality across 3 RCTs, though the authors noted that high-quality evidence remains limited and larger trials are needed.
The UK gap: the NDNS reports that 11% of men and 12% of women aged 19–64 have magnesium intakes below the Lower Reference Nutrient Intake. Serum magnesium is a blunt marker (only 1% of body magnesium is in the blood), so a “normal” serum level does not rule out tissue-level depletion. Optimal serum magnesium for sleep is considered to be 0.85–0.95 mmol/L, above the NHS lower limit of 0.7.
5. Cortisol
Cortisol follows a diurnal rhythm: it should peak within 30 minutes of waking (the cortisol awakening response) and fall steadily through the day, reaching its lowest point around midnight. Melatonin rises as cortisol falls. When this rhythm is flattened or inverted — from chronic stress, overtraining, shift work or HPA-axis dysfunction — the melatonin surge is blunted and sleep onset is delayed.
A 2015 study in Psychoneuroendocrinology demonstrated that elevated evening cortisol predicted longer sleep onset latency and more wake-after-sleep-onset (WASO) episodes in otherwise healthy adults. The relationship was dose-dependent: each standard-deviation increase in evening cortisol was associated with 12 additional minutes of WASO.
A morning fasted blood test measures your peak cortisol. If this is unusually high (>700 nmol/L) or unusually low (<170 nmol/L), it signals HPA-axis dysregulation. Combined with DHEA-S, the cortisol:DHEA-S ratio reveals whether your stress response is in overdrive (high ratio) or exhaustion phase (low cortisol, low DHEA-S).
See our cortisol blood test guide for the full cortisol:DHEA-S framework and what to do at each stage.
6. Testosterone
The relationship between testosterone and sleep runs in both directions. Poor sleep reduces testosterone, and low testosterone impairs sleep quality.
A widely cited 2011 study in the Journal of the American Medical Association found that restricting young healthy men to 5 hours of sleep per night for one week reduced daytime testosterone levels by 10–15%. The effect was equivalent to ageing 10–15 years.
Low testosterone has been independently associated with obstructive sleep apnoea (OSA) in men. The NHS estimates 4% of middle-aged men have OSA, many undiagnosed. If you snore, have morning headaches, and your testosterone is low, OSA should be considered — a referral for a sleep study is appropriate.
In women, testosterone is produced in smaller amounts but still matters for sleep. Falling testosterone during perimenopause contributes to insomnia alongside declining oestrogen and progesterone. See our testosterone blood test guide and menopause blood test guide for more context.
7. Vitamin B12
Vitamin B12 is involved in the synthesis of melatonin via the methylation pathway. B12 acts as a cofactor for the enzyme methionine synthase, which produces S-adenosylmethionine (SAMe) — the methyl donor that converts serotonin to melatonin in the pineal gland.
A 2019 cross-sectional study in Journal of Clinical Sleep Medicine found that higher serum B12 was associated with better sleep quality scores and shorter sleep latency in adults aged 20–80. Conversely, B12 deficiency was associated with circadian rhythm disruption, delayed sleep phase and excessive daytime sleepiness.
The NDNS reports that 6% of UK adults have serum B12 below the deficiency threshold (148 pmol/L), rising to 11% in those over 65 and higher still in vegans and vegetarians.
Timing matters: some sleep specialists recommend taking B12 supplements in the morning rather than evening, as B12 may reduce melatonin production acutely. See our B12 blood test guide for the full interpretation framework.
8. Iron & Transferrin Saturation
While ferritin measures stored iron, serum iron and transferrin saturation measure the iron that is actually available to your tissues right now. The distinction matters for sleep.
Iron is required for the enzyme tyrosine hydroxylase, which converts the amino acid tyrosine into L-DOPA — the precursor to dopamine. In the basal ganglia, dopamine suppresses involuntary movement during sleep. When brain iron is low, dopaminergic signalling is impaired, and the characteristic restless-leg sensations and periodic limb movements of sleep (PLMS) emerge.
The NICE guideline on restless legs syndrome recommends checking serum ferritin in all patients presenting with RLS, and treating with iron supplementation when ferritin is below 75 µg/L. Transferrin saturation below 20% is an additional red flag for functional iron deficiency affecting the brain, even when serum ferritin is borderline normal.
If your ferritin and iron studies are both low and you experience restless legs, leg jerking at night or the urge to move your legs when lying down, iron supplementation is first-line treatment. See our iron deficiency guide for the complete iron panel interpretation.
NHS vs Optimal Ranges for Sleep Quality
The NHS “normal range” is designed to detect disease. The optimal range for sleep quality is narrower and often higher. The table below summarises the difference for each of the eight biomarkers in this guide.
| Biomarker | NHS “Normal” | Optimal for Sleep | Source |
|---|---|---|---|
| Ferritin | 30–400 µg/L | ≥75 µg/L | Sleep Med Rev 2013 |
| TSH | 0.27–4.2 mIU/L | 1.0–2.5 mIU/L | JCEM 2019 |
| Vitamin D | ≥25 nmol/L | 75–125 nmol/L | Nutrients 2018 |
| Magnesium | 0.7–1.0 mmol/L | 0.85–0.95 mmol/L | JRMS 2012 |
| Cortisol (AM) | 170–700 nmol/L | 280–500 nmol/L | Psychoneuroendocrinology 2015 |
| Testosterone (M) | 8.6–29 nmol/L | 15–25 nmol/L | JAMA 2011 |
| Vitamin B12 | ≥148 pmol/L | 300–600 pmol/L | JCSM 2019 |
| Transferrin sat. | 15–50% | ≥25% | NICE CKS RLS |
What Your GP Tests for Sleep Problems vs Helvy
If you go to your GP with insomnia, the first response is usually sleep hygiene advice and possibly a short course of CBT-i (cognitive behavioural therapy for insomnia). Blood tests are often requested only if symptoms persist — and the standard panel is typically TSH, FBC and perhaps ferritin. This misses several high-value markers.
| Marker | GP (typical) | Helvy Essential | Helvy Performance |
|---|---|---|---|
| TSH | ✓ | ✓ | ✓ |
| Free T4 / Free T3 | — | — | ✓ |
| FBC | ✓ | ✓ | ✓ |
| Ferritin | Sometimes | ✓ | ✓ |
| Vitamin D | — | ✓ | ✓ |
| Magnesium | — | ✓ | ✓ |
| Cortisol | — | — | ✓ |
| Testosterone | — | — | ✓ |
| DHEA-S | — | — | ✓ |
| Vitamin B12 | Sometimes | ✓ | ✓ |
| Iron studies | — | ✓ | ✓ |
| hs-CRP | — | — | ✓ |
The Helvy Performance panel (£149) covers all 8 sleep-relevant biomarkers in this guide plus 8 additional markers for metabolic and hormonal health.
5 Sleep-Disrupting Result Patterns
Blood test results rarely exist in isolation. These are five common patterns where multiple markers combine to disrupt sleep.
Pattern 1: The restless sleeper
Low ferritin + low transferrin saturation + normal haemoglobin.
Iron stores are depleted but you are not yet anaemic. You experience restless legs, periodic limb movements, and fragmented sleep. The FBC looks normal, so a GP may dismiss the iron connection. Ferritin below 75 µg/L with transferrin saturation below 20% is the classic profile. Iron supplementation (ferrous sulphate 200 mg, alternate days, with vitamin C) typically improves RLS symptoms within 4–6 weeks.
Pattern 2: The wired-but-tired
High evening cortisol + low DHEA-S + low magnesium.
Your stress response is stuck in overdrive. The cortisol:DHEA-S ratio is elevated, blunting melatonin onset. Low magnesium amplifies the problem by reducing GABA activity. You feel exhausted during the day but wired at bedtime. Common in people with demanding jobs, heavy training loads, or chronic anxiety. Addressing magnesium (glycinate or threonate, 200–400 mg before bed), stress management and training load reduction often helps within 2–4 weeks.
Pattern 3: The foggy riser
High TSH + low vitamin D + low B12.
Subclinical hypothyroidism slows your metabolism, reducing core temperature regulation and impairing slow-wave sleep. Compounded by vitamin D and B12 deficiency, you sleep a lot but wake unrefreshed, with brain fog that persists until midday. Your GP may say your TSH is “borderline normal” at 4.0 mIU/L, but combined with low D and B12, the sleep impact is multiplicative. Treat each deficiency individually and retest in 8–12 weeks.
Pattern 4: The night waker (male)
Low testosterone + elevated hs-CRP + low vitamin D.
Low testosterone in men over 35 is associated with increased wake-after-sleep-onset and reduced sleep efficiency. When combined with elevated inflammatory markers (hs-CRP above 3 mg/L) and low vitamin D, the pattern suggests systemic inflammation driving both hormonal and sleep dysfunction. Investigate sleep apnoea (ask about snoring, morning headaches, witnessed apnoeas). Address inflammation through exercise, diet and weight management. Testosterone replacement is a clinical decision — see your GP or endocrinologist.
Pattern 5: The perimenopausal insomniac
Fluctuating FSH + declining oestradiol + low magnesium + low ferritin.
Perimenopause disrupts sleep through multiple pathways: vasomotor symptoms (hot flushes, night sweats), falling progesterone (a natural sedative), and often co-existing iron and magnesium depletion from years of menstruation. A blood test clarifies which pathways are active. HRT addresses the hormonal component; iron and magnesium supplementation addresses the nutritional. See our menopause blood test guide for the full hormonal picture.
Evidence-Based Interventions by Biomarker
Once you have your results, these are the highest-evidence interventions for each biomarker. Always discuss supplementation with your GP or pharmacist, particularly if you take medications.
Low ferritin
Ferrous sulphate 200 mg on alternate days with vitamin C. Avoid taking with tea, coffee or dairy. Retest at 8–12 weeks. NICE recommends treating until ferritin reaches ≥50 µg/L.
Source: NICE CKS Iron Deficiency
Abnormal TSH
TSH above 4.5 with symptoms warrants GP review and possible levothyroxine trial. TSH below 0.3 needs urgent investigation for hyperthyroidism.
Source: NICE NG145
Low vitamin D
NHS recommends 400 IU/day as maintenance; for deficiency (<25 nmol/L), loading doses of 3,000–4,000 IU/day for 8–12 weeks are common practice. Take with fat for absorption. Retest at 3 months.
Source: NICE CKS Vitamin D
Low magnesium
Magnesium glycinate or threonate 200–400 mg before bed. Glycinate has mild sedative properties; threonate crosses the blood-brain barrier more effectively. Avoid magnesium oxide (poor absorption).
Source: BMC Complement Med Ther 2023
High cortisol
Address root cause: reduce training volume if overtrained, treat anxiety, improve sleep environment. Ashwagandha (600 mg/day) has moderate evidence for reducing cortisol. Retest at 8 weeks.
Source: J Ethnopharmacol 2014
Low testosterone
Optimise sleep, exercise (resistance training), body composition and vitamin D first. If total T remains below 8 nmol/L with symptoms, GP referral for endocrinology assessment.
Source: BSSM guidelines
Low vitamin B12
Oral cyanocobalamin 1,000 µg/day or hydroxocobalamin IM injections if malabsorption suspected. Take in the morning (may reduce melatonin acutely). Retest at 8 weeks.
Source: BSH guidelines
Low iron / transferrin sat.
Same protocol as low ferritin. If transferrin saturation is below 16%, serum iron below 10 µmol/L, investigate cause (dietary, GI blood loss, coeliac). GP referral if refractory.
Source: NICE CKS Iron Deficiency
Frequently Asked Questions
Can a blood test really tell me why I can't sleep?
A blood test cannot diagnose insomnia itself — insomnia is a clinical diagnosis based on symptoms. But it can reveal underlying causes like thyroid dysfunction, iron deficiency, vitamin D depletion or hormonal imbalances that directly impair sleep quality. Fixing the cause often fixes the sleep.
Should I get a blood test before trying sleeping tablets?
Yes. NICE recommends investigating secondary causes of insomnia before prescribing hypnotics. If your poor sleep is driven by low ferritin, subclinical hypothyroidism or vitamin D deficiency, a sleeping tablet treats the symptom while the root cause persists.
Do I need to fast before a sleep blood test?
A morning fasted sample (before 10am, water only from midnight) gives the most accurate cortisol, iron and glucose readings. Thyroid, B12, vitamin D and ferritin do not require fasting but are best measured at the same time for consistency.
How long after supplementing should I retest?
Most biomarkers need 8–12 weeks to show meaningful change. Ferritin and vitamin D are slow to shift; thyroid on levothyroxine needs 6–8 weeks. Don't retest too early — you'll waste money on results that haven't had time to change.
Can low vitamin D cause insomnia?
Observational studies consistently show an association between low vitamin D (below 50 nmol/L) and poor sleep quality. Whether supplementation reliably improves sleep is less clear — but correcting a deficiency is recommended regardless, and some people report significant sleep improvement.
Which Helvy panel should I choose for sleep problems?
The Performance panel (£149) covers all 8 biomarkers in this guide — ferritin, TSH, vitamin D, magnesium, cortisol, testosterone, B12 and iron studies. The Essential panel (£129) covers ferritin, TSH, vitamin D, B12, magnesium and iron but not cortisol or testosterone.
FIND OUT WHAT'S KEEPING YOU AWAKE
Test the 8 biomarkers that affect your sleep.
The Helvy Performance panel includes ferritin, thyroid, vitamin D, magnesium, cortisol, testosterone, B12 and iron studies — everything in this guide. Results in 5 days, reviewed by a GP.
Order your testBy Helvy · Medically reviewed