Lethargy

Lethargy describes sustained physical and mental sluggishness that is disproportionate to activity and not relieved by ordinary rest. It differs from ordinary tiredness in persistence and in its resistance to sleep. Most cases resolve to a short list of causes. Sleep is the dominant one: insufficient duration, but more often unrecognised obstructive sleep apnoea, which produces unrefreshing sleep and daytime heaviness in people who believe they slept adequately. Iron deficiency is the next most common, and it matters that fatigue appears well before haemoglobin falls, so ferritin must be checked rather than relying on a normal full blood count. Hypothyroidism, vitamin B12 and vitamin D deficiency, and poorly controlled diabetes are the other routinely checkable causes. Depression presents as lethargy at least as often as it presents as sadness, particularly in men and older adults, and anhedonia is the more reliable clue than low mood. Medications are heavily implicated — beta blockers, antihistamines, sedatives, opioids and some antidepressants. Post-viral fatigue, including after COVID-19, is well recognised and typically improves over months. Lethargy with weight loss, night sweats, fever or lymph node swelling is a different picture requiring prompt assessment.

• •Persistent heaviness and low energy not relieved by rest
• •Waking unrefreshed despite adequate time in bed
• •Reduced physical stamina and effort tolerance
• •Mental slowness alongside physical sluggishness
• •Needing to nap during the day
• •Reduced interest in usual activities
• •Muscle weakness or heaviness in the limbs
• •Feeling cold, dry skin and constipation suggesting thyroid causes
• •Breathlessness on exertion suggesting anaemia
• •Snoring or witnessed pauses in breathing

• •Obstructive sleep apnoea and other sleep disorders
• •Insufficient or irregular sleep, including shift work
• •Iron deficiency, with or without anaemia
• •Hypothyroidism
• •Vitamin B12, folate and vitamin D deficiency
• •Depression and anxiety
• •Poorly controlled diabetes
• •Medications: beta blockers, antihistamines, sedatives, opioids, some antidepressants
• •Chronic kidney or liver disease
• •Heart failure
• •Coeliac disease and malabsorption
• •Post-viral fatigue including long COVID
• •Chronic fatigue syndrome and fibromyalgia
• •Alcohol use
• •Underlying malignancy, particularly with weight loss or night sweats

Energy production depends on oxygen delivery, thyroid-set metabolic rate, mitochondrial substrate availability and adequate restorative sleep. Iron deficiency impairs both oxygen carriage and mitochondrial enzyme function, which is why symptoms precede anaemia. Sleep apnoea fragments sleep architecture and causes repeated hypoxaemia without the person waking enough to notice. Thyroid hormone deficiency slows metabolism globally. Systemic inflammation from infection or chronic disease produces cytokine-mediated sickness behaviour, an evolved conservation response that manifests as lethargy.