When I get really tired I start forgetting nouns. For example, "my notebook" becomes "that thing I write in." A cup becomes "that thing I drink out of." And even people I just talked to become "that guy" or "that girl."
Thursday I was really tired, so I pulled out "that thing I write with" and, while standing next to "that thing I walk along," dedicated the day to the people whose names I could not remember, which was pretty much everyone.