High time resolution Schumann resonance (SR) records are analyzed at a midlatitude (Nagycenk, 47.6°N, 16.7°E, Hungary) and a north polar (Hornsund, 77°N, 15.5°E, Spitsbergen) station from the point of view of the day-night asymmetry of the Earth-ionosphere cavity. The vertical electric field component, EZ, at Nagycenk in quasi-minute time resolution exhibits jump-like increases of SR amplitudes between the local ionospheric and surface sunrise times and sharp decreases between the local surface and ionospheric sunset times. These amplitude variations depend on frequency, increase with increasing mode number, and occur simultaneously in the three SR modes studied here. The duration of the sharp frequency-dependent amplitude changes is generally less than 30 min. The accurate timing (“clock-like accuracy”) of these sharp SR amplitude variations of about 12–25% in the local sunrise/sunset periods and their frequency dependence make these changes distinguishable from the amplitude variations related to the lightning source properties and strongly suggest an ionospheric origin for these sharp amplitude variations. The signature of the day-night asymmetry of the Earth-ionosphere cavity can also be found at Hornsund in the two short spring and autumn periods with alternating day and night periods every day, in the form of an enhanced day-night contrast of the SR amplitudes with consistent frequency dependence for the first three SR modes.