I am getting a night guard for my jaw, is it ok that i wear it in the day ?

If you have TMJ then your splint must be worn 24/7. If your dentist tells you otherwise then he/she does not know how to treat TMJ and is simply taking a shotgun approach to "treatment." BTW, grinding does not cause TMJ as many people and dentists who do not understand TMJ disorders believe but it can aggravate an existing TMJ condition. However, if your appliance was made because you grind your teeth at night and you do not have a jaw probelm, it will do no harm to wear it during the day.