Does magnesium help your muscles and sleep?

Short answer

It did nothing for muscles and only a little for sleep. Strength did not change in people who train, night leg cramps in older adults did not drop, and sleep came only about 17 minutes faster.

Does magnesium help your muscles and sleep?
'Magnesium → relaxed muscles → fewer cramps, better sleep' — does this formula hold?
Many people who train take magnesium, saying it relaxes muscles, stops cramps and helps them fall asleep. We checked three things in turn: strength, cramps and sleep.
Strength didn't change in people who train
Benefits showed up only in people likely to be low
An analysis that pooled 14 trials found that in 215 athletes and active people, magnesium did not improve strength or power. Benefits appeared only in groups likely to run low on magnesium, such as older adults and heavy drinkers.
After 9 days, performance actually dropped
Peak endurance fell 7%
15 people took 300 mg of magnesium chloride twice a day for 9 days. Their maximum oxygen uptake fell from 44.4 to 41.3, about 7% lower, and power in a 30-second sprint dropped about 5%. 10 km time trial results did not differ.
What about cramps?
Night leg cramps in older adults didn't drop
An analysis of 11 trials with 735 people found that in 307 older adults with night leg cramps, magnesium changed weekly cramps by just 0.18 versus placebo — essentially the same. Not a single trial tested magnesium for cramps during exercise.
Sleep came a little faster
17 minutes quicker to fall asleep, no gain in total sleep
An analysis of 3 trials with 151 older adults with insomnia found magnesium cut the time to fall asleep by about 17 minutes. Total sleep rose 16 minutes, but that was not a meaningful difference.
In younger adults, the effect was small too
Insomnia scores dropped only a bit more
155 poor sleepers aged 18-65 took 250 mg a day for 4 weeks. Their insomnia score fell 3.9 points, versus 2.3 points on placebo. The researchers themselves called the effect small, and people who normally ate little magnesium improved more.
Bottom line: nothing for muscles, a little for sleep
If your diet covers it, there was nothing more to gain
Strength did not change in people who train, and night leg cramps in older adults did not drop. Sleep came about 17 minutes faster, and a 2026 review of 12 trials did not recommend it as an insomnia treatment. Benefits showed up only in people who eat little magnesium or are likely to be low.