I would concur with your Trainer friend, Magnesium is useful as it is a precursor to malatonin, the sleep hormone. I will add this if OK…Any form of magnesium can be used to attenuate a magnesium deficiency, except magnesium L-threonate, since it contains less elemental magnesium per dose. Magnesium citrate is a good choice for supplementation, try going with 200-400mg per day and aim to consume with food to aid absorption.
If I were you then I would go for Magnesium citrate at a dose of 200mg to begin with and see how it goes. Some can cause a laxative effect but that is generally when magnesium oxide or magnesium chloride is used.
Another empirical method of improving sleep is the combination of Apple cider vinegar (Bragg’s is the one to go for), honey and hot water. Cal Orey, author of “The Healing Powers of Vinegar,” recommends combining 2 teaspoons of apple cider vinegar with honey as an effective tonic to treat insomnia. However I have a lot of time for Tim Ferris who personally insists that 1 cup hot water, 2 tbsp organic apple cider vinegar and 1 tbsp organic honey has real tranquillising effects of a night time.
We don’t know why it works…it just does (you won’t find that statement in a peer reviewed science journal any time soon haha).