Night effect can be a combination of both, but the primary cause is due to polarity change of the signal when reflected from the ionosphere. Ground and sky wave arrive at different times, this is of no consequence to the ADF receiver but the angle of incidence of the skywave is. Other transmissions within the pass band, not necessarily NDBs, as few are on the same frequency(synchronous transmissions), can add to the problems. Multipath may also occur to the groundwave, called terrain effect.
Dip and Night effect are both in their simplest form, due to a rotation of the incoming signal relative to the antenna.