What Counts as Night Sweats?
True night sweats are episodes of drenching perspiration during sleep — severe enough to soak through nightclothes and bedding — that are not explained by an overheated room or too many blankets. They are distinct from simply feeling warm at night. Night sweats are a common complaint, estimated to affect 40% of adults at some point, but identifying the cause is important because it ranges from entirely benign to occasionally serious.
Most Common Causes
Menopause and Perimenopause
The most common cause of night sweats in women aged 40–55. Declining oestrogen disrupts the hypothalamic "thermostat," causing hot flashes — sudden waves of heat — that frequently occur during sleep. Up to 80% of perimenopausal women experience hot flashes; they typically last 1–5 years but can persist for more than 10 years in some women. Hormone replacement therapy (HRT) is the most effective treatment, reducing frequency by 75%.
Medications
Many commonly prescribed drugs cause night sweats as a side effect:
- Antidepressants — especially SSRIs (sertraline, fluoxetine) and SNRIs (venlafaxine)
- Hormone therapies — tamoxifen, GnRH agonists (used for prostate cancer or endometriosis)
- Antihypertensives — beta-blockers, calcium channel blockers
- Antipyretics taken at night — aspirin, ibuprofen, paracetamol (causing a rebound in temperature)
- Opioids and methadone
Anxiety and Stress
The sympathetic nervous system activation that drives anxiety also triggers sweating. People with anxiety disorders, PTSD, or high baseline stress levels frequently report night sweats that improve with treatment of the underlying anxiety.
Alcohol and Substance Use
Alcohol dilates blood vessels and can cause flushing and sweating during sleep. Alcohol withdrawal — even in people who drink heavily but do not consider themselves dependent — produces prominent night sweats 6–24 hours after the last drink.
Low Blood Sugar (Nocturnal Hypoglycaemia)
In people with diabetes on insulin or sulfonylureas, blood glucose can drop during sleep, triggering a stress hormone (epinephrine) response that causes sweating. Waking with a headache or feeling shaky is suggestive.
Serious Causes to Rule Out
The following conditions are less common but important to exclude — particularly when night sweats are severe, persistent, and accompanied by other symptoms:
- Infections — tuberculosis (classically), HIV, endocarditis, or other chronic infections cause constitutional symptoms including drenching night sweats, fever, and weight loss
- Lymphoma (Hodgkin and Non-Hodgkin) — night sweats are part of the "B symptoms" triad (along with fever and unexplained ≥10% weight loss) that indicate systemic disease activity and carry prognostic significance
- Other cancers — certain solid tumours, especially carcinoid tumours, produce hormones that cause flushing and sweating
- Hyperthyroidism — elevated metabolic rate causes heat intolerance, sweating, weight loss, and palpitations
- Sleep apnoea — arousals from apnoeic episodes can cause sweating
When to See a Doctor
See your GP if night sweats are severe, occurring most nights, and accompanied by: unexplained weight loss, persistent fever, enlarged lymph nodes, fatigue out of proportion to activity, or if you have risk factors for TB or HIV. Investigations typically include blood count, inflammatory markers (ESR, CRP), TSH, HIV test, glucose, chest X-ray, and sometimes imaging.