I hav been studying in the night and sleep in the day for a long time, is it bad for my health? How to change? Thx

If it works for you. If this schedule works with the rest of your life, it may not be "bad." some people prefer to be awake/work at night and asleep in the day. Are you getting enough (at least 7-8 hours) of uninterrupted sleep each 24 hours? How are your relationships with family and others? Do you exercise, and eat healthily? If any of these are problems, your habit may need to change. The shift would be gradual.