Is it better to take magnesium in the evening?

Magnesium has a biological basis for being linked to sleep. The mineral dampens the excitability of nerve cells and plays a role in the production of melatonin, the sleep hormone. Review research describes this mechanism convincingly, but large randomised trials confirming this in humans are not yet available. It is therefore biologically plausible, but not yet a proven clinical effect.

Dietary magnesium deficiency is linked in associative research to poorer adaptation to 'social jet lag' (the difference between your biological and your social sleep time) and a higher risk of rhythm disturbances. Supplementation then logically makes sense, but whether you do that in the morning, during the day or in the evening is not something that research addresses.

And that is precisely the core of the answer to your question: none of the available studies compare evening intake with intake at another time of day. The widely heard recommendation to take magnesium in the evening has a logical rationale behind it (sleep-promoting effects), but clinical evidence for that specific timing is entirely absent.

A small study in people with high blood pressure showed that magnesium, divided across three moments during the day, measurably lowered blood pressure. This involved 17 patients, making it too small for firm conclusions, and the timing was explicitly spread out and not specifically in the evening.

From a practical standpoint: if you are taking magnesium for sleep, one hour before bedtime is a reasonable choice that fits the mechanism. But if you prefer to take it in the morning or with a meal, there is no evidence that you will miss out on a sleep effect by doing so. Stomach complaints are a well-known side effect at higher doses; taking it with a meal helps to reduce that.