i have a friend who is taking 420 mg of magnesium oxide twice a day and he thinks that it is the magnesium that makes him feels tired and sleepy all day. he sleeps around 11 pm each night and gets up around 3 or 4 in the morning. he is taking amlodipine 5 mg daily . aside from his cholesterol pill. aspirin 81 mg. vit d 1000 iu and on warfarin 5mg x 4 days and 7.5 m,w,f his blood level for warfarin was good.