This article is for general information only and is not medical advice. If you have a health condition, take prescribed medication or are pregnant, speak to your doctor or pharmacist before changing your supplements.
The short answer
Some people do wake up groggy after taking magnesium the night before, although it hasn’t been properly studied. When it happens it is usually linked to a bedtime dose, a larger dose than you need, magnesium glycinate, or a sleep blend that contains more than magnesium. Moving the dose earlier, lowering it or switching product fixes it for most people.
This article is about feeling foggy the morning after. If you feel drowsy during the day soon after a dose, read magnesium is making me sleepy and drowsy.
What the sleep research shows
Magnesium is widely taken for sleep, but the evidence is thin. A 2021 review found only three small trials in older adults with insomnia. People taking magnesium fell asleep about 17 minutes sooner than those on a placebo, and the authors rated the evidence as low quality.
None of the trials measured how people felt the next morning. So there is no research that tells us how common next-day tiredness is, or exactly why it happens. The explanations below are the most plausible ones.
Why you might feel groggy the next morning
The glycine in magnesium glycinate
Glycinate is magnesium bound to glycine, an amino acid that has been studied as a sleep aid in its own right. Many people choose it for that reason. If you are sensitive to it, the same calming effect may still be there when the alarm goes off.
A sleep blend, not plain magnesium
Night-time products often add melatonin, L-theanine, 5-HTP, valerian, chamomile or lavender. Several of these can cause morning drowsiness, melatonin in particular. Check the label before blaming the magnesium.
Dose and timing
A large dose taken right at bedtime is the most common pattern in the messages we receive. The NHS says adults need about 270 mg (women) to 300 mg (men) of magnesium a day from all sources, much of which comes from food.
Your sleep has changed
If you are falling asleep earlier or sleeping more deeply than you have for a while, waking can feel heavier at first. A disturbed night from an upset stomach has the same result for the opposite reason. Loose stools are the best-established side effect of magnesium, and they don’t keep office hours.
Something else entirely
Alcohol, a late night, a cold coming on and many medicines cause morning grogginess. If it only happens now and then, magnesium may not be involved.
What to try
Change one thing at a time and give it several nights.
- Take it earlier. Have it with your evening meal, not at bedtime.
- Lower the dose. Halve it for a week.
- Check the label. If your product is a sleep blend, try plain magnesium for comparison.
- Try another form. If you take glycinate, citrate or malate may suit you better. Citrate is more likely to loosen stools.
- Skip a few nights. If mornings are no different without it, look at other causes.
- Keep a simple diary. Note the dose, the time and how you feel on waking. A pattern usually shows within a week or two.
If you feel drowsy, don’t drive or use machinery until it has passed.
When to speak to your GP
- You have been tired for a few weeks and can’t explain it. The NHS advises getting this checked, as tiredness has many causes including anaemia, thyroid problems, diabetes and sleep apnoea.
- You have kidney disease. The kidneys clear excess magnesium, so supplements can build up and cause drowsiness and weakness.
- You take regular medicine and want to take magnesium alongside it.
Check how your supplements fit together
If you take magnesium with other minerals or vitamins at night, they may affect each other. Tap magnesium on our free Mineral Interaction Wheel to see what it works with and what it may compete with. For other side effects, see our magnesium side effects guide.
Sources
- NHS. Vitamins and minerals: Others (magnesium).
- Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis. BMC Complementary Medicine and Therapies, 2021.
- Yamadera W, et al. Glycine ingestion improves subjective sleep quality in human volunteers. Sleep and Biological Rhythms, 2007.
- NHS. Tiredness and fatigue.