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Thirteen of 20 normal adults given 10 milligrams of dextroamphetamine exhibited paradoxical drowsiness accompanied by lowered electrical brain activity (contingent negative variation, or CNV) in the first hour post-drug. During this period, seven subjects showed behavioral alertness and increased CNV amplitude. Both groups of subjects showed heightened alertness 2 and 3 hours post-drug. Amphetamine is not a simple stimulant of the central nervous system but can also act as a depressant.
Tecce et al. (1974) studied this question.