At home teeth whitening done by dentist?

I am picking up trays and the whitening from my dentist next week to do at home. I need to put in the trays 3 hours a day for 3 weeks. Did someone ever do this? Can I open my mouth when the trays are in or do I have to have my mouth shut and not move for 3 hours? I have sinusitis so my nose is really congested. I'm wondering how will I breathe if I can't open my mouth?