Members of the RMT union have walked out on strike in the ongoing row over the safety of driver-only operated trains.
Northern said the majority of trains would still run between 7am and 7pm but buses will replace trains on some routes.
A further strike has also been planned for Saturday, when Northern said the majority of trains would run between 7am and 5pm.
A Northern spokesman said: "As the overall number of trains running will be reduced, we expect trains and any replacement buses we operate to be extremely busy. Please allow extra time for journeys, plan carefully and consider whether travel is necessary.
"During the RMT strike action we expect all services to be busy, especially in the morning and evening peak periods, and advise you to allow extra time to travel."
RMT general secretary Mick Cash said: “It is a tribute to the determination and professionalism of RMT members on Arriva Rail North that they have remained rock solid for over a year now in what is a battle to put public safety before private profit.
"The next phase of action on Thursday and Saturday goes ahead exactly as planned as the company refuse point blank to engage in talks.
"Northern Rail want to run half a million trains a year without a safety critical guard on board in a move that would wreck both safety and access to services and they should listen to their front-line staff and pull back from that plan immediately.
“We have seen again this week that Arriva are not capable of running a railway and have chosen to declare war on passengers and staff alike in the drive for increased profits while safety, access and reliability are left to rot.
"RMT has agreed arrangements in Wales and Scotland that enshrine the guard guarantee. If it's good enough for Wales and Scotland to have safe rail services it should be good enough for the rest of Britain.
“We thank the public for their support and understanding throughout this dispute over rail safety and access and the union remains ready for talks.”