What You Need to Know About Magnesium and Your Sleep
psychologytoday.com
psychologytoday.com
Some forums and health websites suggest significantly higher doses, but I think that's a bad idea. Your body will tightly regulate magnesium levels within a specific range. Large doses (500mg+ per day) will just make your body work harder to eliminate the extra magnesium.
Also consider that Gwern famously self-experimented with magnesium on two separate occasions, and found a long-term trend toward decreased performance: https://www.gwern.net/nootropics/Magnesium It's anecdotal, of course, but it's not hard to imagine that the megadoses of magnesium could cause electrolyte imbalances or other disturbances if continued long term.
Keep it low dose and keep it safe.
Don't over do it.
Epsom is magnesium sulphate heptahydrate (MgSO4·7H2O). The molar mass ratio of epsom to Mg is about 10. Thus, in order to get 350 mg of elemental magnesium, you would take about 3.5 g of epsom salt, which is a little less than a teaspoon.
The taste of epsom is completely hidden by almost anything you put it in. I mix it into tea or juice.
If taken on an empty stomach, the oxide and carbonate forms are less available because the Mg cation won't dissociate. However if they're taken with food, these chemicals dissolve in your gastric acid, and the magnesium will be absorbed the same as any other magnesium salt.
Physical packaging can greatly affect absorption; for example, extended release formulations often show very poor availability. But minimally packaged magnesium salts are all pretty much the same in terms of total uptake.
Here's a good summary:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5652077/ - "Intestinal Absorption and Factors Influencing Bioavailability of Magnesium-An Update-"