Fewer off-off-peak trains will serve Uckfield and Buxted from February 22, when a new timetable comes into operation.
The changes are designed to better match services to demand and are a reaction to the ongoing coronavirus pandemic.
People should stay at home and only travel for work if absolutely necessary.
From February 22, the usual early-morning trains, starting with the 05:31 from Uckfield, continue to run to London Bridge.
The first alteration is with the 09:33 which will terminate at East Croydon where passengers must change for London.
The10:33 runs as normal but the services are then two-hourly until the 16:33.
London Bridge departures become two-hourly after the 09:07 until the 16:07 when the normal hourly service resumes.
Uckfield Line services were reduced earlier in the pandemic because of the drop in demand.
Buxted trains are four minutes later on services to the capital.
Train operator Southern warns that timetables are still be updated and people should check before they travel.