Insomnia & Sleep Issues: Which Labs to Check When Sleep Won't Come

Overview.

Chronic insomnia affects 10-15% of adults and has profound health consequences — increasing risk of cardiovascular disease, diabetes, depression, and all-cause mortality. While most insomnia is behavioural (poor sleep hygiene, stress, anxiety), several medical conditions cause or worsen insomnia through distinct mechanisms: thyroid dysfunction, iron deficiency (restless legs), cortisol dysregulation, magnesium depletion, and blood sugar instability. A targeted lab workup identifies these treatable causes.

Insomnia is difficulty initiating sleep, maintaining sleep, or early morning awakening, occurring at least 3 nights per week for at least 3 months, despite adequate sleep opportunity. Sleep-onset insomnia (can't fall asleep) and sleep-maintenance insomnia (waking at 2-4 AM) have different differential diagnoses and may point to different medical causes.

Prevalence: 10-15% of adults meet criteria for chronic insomnia disorder. Up to 30-40% report insomnia symptoms at any given time. Women are 1.5x more likely to have insomnia than men. Prevalence increases with age.

What to test.

First-line tests

• TSH — Both hyperthyroidism (causes insomnia through hyperarousal, tachycardia, anxiety) and hypothyroidism (causes daytime fatigue but paradoxically disrupts sleep architecture) affect sleep quality.
• Ferritin — Iron deficiency is the most common modifiable cause of restless legs syndrome (RLS) — an urge to move the legs that worsens at rest and at night, preventing sleep. Ferritin <75 ng/mL is the treatment threshold for RLS (lower than the general deficiency threshold).
• CMP (Metabolic Panel) — Screens for calcium abnormalities (hypercalcaemia causes insomnia), liver dysfunction (hepatic encephalopathy disrupts sleep-wake), kidney dysfunction (uraemia causes restless legs), and glucose abnormalities.
• CBC — Anaemia causes tachycardia and restlessness. Polycythaemia (high red cells) may suggest sleep apnoea (chronic hypoxia).

Second-line tests

• Magnesium (RBC Magnesium preferred) — Magnesium is a natural GABA receptor agonist and nervous system relaxant. Deficiency is associated with insomnia, anxiety, and muscle cramps that disturb sleep. RBC magnesium is more accurate than serum.
• Cortisol (AM and/or salivary cortisol rhythm) — Cortisol should be lowest at bedtime and peak at waking. Disrupted cortisol rhythm (high evening cortisol) prevents sleep onset. Cushing's syndrome causes severe insomnia.
• Fasting Glucose + HbA1c — Nocturnal hypoglycaemia causes 2-4 AM awakenings with anxiety, sweating, and hunger. Blood sugar instability disrupts sleep architecture.
• Vitamin D — Vitamin D receptors are present in sleep-regulating brain regions. Deficiency is associated with shorter sleep duration and poorer sleep quality in observational studies.

Specialized tests

• Testosterone (Men) — Low testosterone in men is associated with sleep fragmentation and sleep apnoea. Conversely, sleep apnoea suppresses testosterone. The relationship is bidirectional.
• FSH / Estradiol (Perimenopause) — Perimenopausal hormone fluctuations cause vasomotor symptoms (night sweats, hot flashes) that severely disrupt sleep. Consider in women 40-55 with new-onset insomnia.

Common causes.

• Hyperthyroidism — Insomnia with racing mind, palpitations, anxiety, weight loss, heat intolerance
• Iron Deficiency / Restless Legs — Irresistible urge to move legs at rest, especially at bedtime. Temporarily relieved by movement.
• Perimenopause — Night sweats and hot flashes causing multiple awakenings. New-onset in women 40-55.

Diagnostic patterns.

• Low ferritin (<75 ng/mL) + restless legs at bedtime + sleep-onset insomnia — likely Iron deficiency-driven restless legs syndrome. Next step: Iron supplementation (ferrous bisglycinate or IV iron); target ferritin >75 ng/mL
• Low TSH + elevated FT4 + insomnia + palpitations + weight loss — likely Hyperthyroidism. Next step: Endocrinology referral for thyroid workup and treatment
• Normal labs + poor sleep hygiene + response to CBT-I — likely Primary insomnia (behavioural). Next step: CBT-I (cognitive behavioural therapy for insomnia) is first-line treatment
• 2-4 AM awakenings + anxiety/hunger on waking + elevated fasting insulin — likely Nocturnal hypoglycaemia / reactive blood sugar instability. Next step: Protein-rich snack before bed; reduce refined carbohydrates at dinner; assess HOMA-IR

Lifestyle.

Non-medical causes

• Poor sleep hygiene (irregular schedule, screen time, bedroom environment)
• Caffeine after 12 PM (caffeine half-life is 5-6 hours)
• Alcohol (sedates initially but fragments sleep in second half of night)
• Stress, anxiety, depression (bidirectional with insomnia)
• Shift work and jet lag
• Medications: corticosteroids, beta-agonists, SSRIs, stimulants, diuretics
• Chronic pain
• Environmental factors (noise, light, temperature, partner disturbance)

Considerations

• Consistent wake time 7 days/week (the single most important sleep hygiene intervention)
• No caffeine after 12 PM (or eliminate entirely as a 2-week trial)
• Blue light reduction 1-2 hours before bed
• Cool bedroom temperature (18-20°C / 64-68°F)
• CBT-I (cognitive behavioural therapy for insomnia) is more effective than medication long-term
• Regular exercise (but not within 2-3 hours of bedtime)
• Alcohol-free trial for 2 weeks to assess sleep quality impact

FAQs.

Red flags.