Temporarily tightens blood vessels, which reduces under-eye puffiness for a few hours.
Caffeine constricts blood vessels and mildly reduces fluid accumulation, which is a genuine but strictly temporary effect on morning puffiness. It does nothing for dark circles caused by pigment or by the shadow of hollowing, which together account for most cases.
skinfrndly is educational and is not medical advice. Recommendations are generated from the answers you gave and general dermatological guidance — they cannot account for your full history. For persistent or severe skin conditions, or before starting anything new during pregnancy, speak to a doctor or dermatologist.