My experience is that cheap magnesium oxide tablets act like sleeping pills for me and can be too sedating if I take too many. I've also tried more expensive pills that claim to be more bioavailable but I'm not impressed.
Magnesium oxide is barely absorbed at all - as in studies show absorption around 4%. [1] It's mainly used as a laxative. The only reason it's sold in supplements as far as I can tell is it's cheap and supplements are roughly speaking entirely unregulated in America.
If you actually want to get the magnesium into yourself, you should use a chelated form like citrate, glycinate, taurate or L-threonate. These are in the 70-100% bioavailability range. L-threonate in particular has been shown to readily cross the blood-brain barrier. Not worth wasting your time and money on oxide or even worse somehow, stearate.
Also true for Zinc. I think half the reason the supplement companies do this is that the oxides are also physically small, so they can get a given dosage in fewer/smaller pills. This is especially true for multivitamins.
I know that's what the literature says but I have tried MgO and alternatives and don't see a difference which might point to it being a placebo effect.
An actual expert can fact check me but as far as I'm aware that's due to how magnesium sits within binding site within the NMDA receptor ion channel as a blocking / gating factor. If supplementation actually raises the level of available magnesium beyond the blood brain barrier (when you're perhaps in need of some) then it would affect the way in which that ion channel affects the downstream excitatory effects towards long term potentiation.
I haven't had the sedating effect, but I can concur I've not been impressed with what's available on the market. Many premium brands offer various magnesium formulas aimed at decreasing stress and/or improving sleep, but I've yet to find one that seems to make a difference.
