Leg cramps at night (komura-gaeri): is there a way to prevent them?
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Waking at night with a violent pain in the calf — komura-gaeri in Japanese. More than half of middle-aged and older people experience it, and it is common in pregnant women and after exercise. Many causes are suggested — "not enough water", "lack of minerals", "cold feet" — but much about it is still unclear. Here is what is known, and what to do in the moment.
What to do in the moment
| Step | Details |
|---|---|
| 1. Stretch | Straighten the knee and pull the toes towards the shin (with your hand, or by pressing the sole against a wall). 20–30 seconds, slowly |
| 2. Stand | If you can, stand and put your weight on the cramped leg (stretches the Achilles tendon) |
| 3. Warm and massage | Once the contraction has passed, massage gently and warm. Hard massage can damage the muscle |
| 4. Afterwards | Muscle soreness may last until the next day. If bad, cool it → warm it the next day |
Suggested causes and what to do about them
| Factor | Details | Measures |
|---|---|---|
| Muscle fatigue | Standing work or exercise during the day; weak muscles from lack of exercise | Stretch before bed (calves and hamstrings, 30 seconds × 2–3 each). Light exercise several times a week |
| Dehydration and electrolytes | Sweat, alcohol, diuretics, diarrhoea. Low magnesium, potassium or calcium may play a part (evidence is limited) | Fluids during the day, cut back on evening alcohol, a balanced diet (studies on supplements disagree) |
| Cold | Cold feet at night make the muscles tense | Leg warmers and socks, a hot-water bottle, warm up in the bath before bed |
| Sleeping position | Toes pushed into a fully pointed position (plantar flexion) by the weight of the duvet makes the calf prone to contracting | A light duvet, space at the foot of the bed, sleep on your side |
| Pregnancy | Common in late pregnancy (about 30–50% experience it). Changes in blood flow and minerals | Stretch, keep warm, ask your obstetrician (magnesium preparations are sometimes prescribed) |
| Medicines | Can increase with diuretics, statins (cholesterol drugs), some asthma medicines | If cramps increased after starting a medicine, ask the prescribing doctor (do not stop on your own) |
| Illness | Lumbar spinal stenosis, herniated disc (with numbness), varicose veins, peripheral arterial disease (swelling, pain on walking), diabetes, thyroid, kidney or liver disease | Frequent (several times a week or more), in the daytime too, with numbness, swelling or pain on walking → internal medicine or an orthopaedic clinic (整形外科, seikei-geka) |
- Kampo: shakuyaku-kanzō-tō (芍薬甘草湯): generally described as working within minutes when taken during an attack, and often prescribed by orthopaedic and internal medicine doctors. But it contains liquorice (kanzō), and continuous use can cause low potassium, swelling and raised blood pressure (pseudoaldosteronism), so the rule is to take it as needed, not every day as prevention. Older people need particular care.
- Magnesium: outside pregnancy, studies showing an effect and studies showing none are mixed. Seaweed, nuts, beans and brown rice in the diet are enough. People with reduced kidney function should avoid supplements.
- The bedtime stretch: hands on a wall, one leg back with the heel on the floor, stretch the calf for 30 seconds. Sit with your legs straight, loop a towel around the sole and pull it towards you. 2–3 times each side. Doing it right before bed has been reported to reduce night-time cramps.
- Age: about half of people aged 60 and over experience it at least once a month. Muscle mass and tendon flexibility decline with age, so stretching and light strength training are the core of prevention.
[Verify] Check the rates for older people and pregnant women, and the side-effect statements for shakuyaku-kanzō-tō, against the relevant societies and the package insert. Check the state of research on magnesium against the latest reviews.
Misunderstanding 1｜When it cramps, massage it hard
Hard massage during the contraction can damage the muscle. First "stretch slowly"; massage gently and warm once it has passed.
Misunderstanding 2｜Drink water and you can prevent it
Dehydration is one factor, but not the only one. Drinking a lot right before bed only means more trips to the toilet at night. Combine daytime fluids with stretching and keeping warm.
Misunderstanding 3｜Magnesium supplements prevent it
Results are not consistent, and outside pregnancy the effect is limited. Diet is enough; avoid supplements if kidney function is reduced.
Misunderstanding 4｜Take shakuyaku-kanzō-tō every night to be safe
The liquorice side effects (low potassium, swelling, raised blood pressure) make this a medicine to avoid taking continuously. As needed during an attack is the rule.
Misunderstanding 5｜It is just age, nothing to be done
Several times a week, in the daytime too, or with numbness or swelling can point to back, blood-vessel or metabolic disease, or a drug side effect. Ask an internal medicine doctor at least once.
---
Sources
- Japanese Orthopaedic Association, "Leg cramps (muscle cramps)" [こむら返り（筋けいれん）]
https://www.joa.or.jp/public/sick/
- Ministry of Health, Labour and Welfare (MHLW) e-Health Net, "Exercise and muscle cramps" [運動と筋肉のけいれん]
https://www.e-healthnet.mhlw.go.jp/information/exercise/
- Pharmaceuticals and Medical Devices Agency (PMDA), "Shakuyaku-kanzō-tō package insert (warning on pseudoaldosteronism)" [芍薬甘草湯 添付文書（偽アルドステロン症の注意）]
https://www.pmda.go.jp/
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
This article is not meant to diagnose or manage any specific medical condition. If you are worried about your health, see a doctor rather than relying on your own judgement.
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