I take 200mg magnesium citrate around 1:30-2pm mon-friday (after eating) and don't feel any different, positive or negative. I only took two once but at different times and can't say if it made me drowsy because I took the second just before bed. I didn't feel considerably more tired the following day.
I take it along with 5,000IU of D and 2 fish oil caps (NOW DHA 500)... I like to take them all after eating as it's easier for me to remember. On weekends I may or may not take any of them, harder to remember to do it.