Lithium is used to treat mania which has often been thought of as the opposite of depression.
Other than making people lethargic
(The self-appointed overlord's would love that)
what possible use would it have?
Sounds like you answered your own question.
lithium has been used to treat neurological causes of mania/ hyperactivity in rare cases (we had a retarded hyperactive client who only calmed down after starting lithium).
but lithium in bipolar doesn’t make them sleepy as much as cuts the intensity of cycles so they are not so “high” or “low”