Biden suffers from sundowning, which starts in the early afternoon, most days, for someone with onset of dementia, as Joe sure seems to suffer from. The debate starts at 9PM eastern time, well past the early bird special and his regular bed time.
His campaign staff can pump him up with whatever drug they think will work, but the only way to really keep him from displaying his dementia over the course of a 90 minute evening debate would be to hold it on a Saturday morning at about 9AM. Tuesday evening at 9PM won't cut it.