Grand jury is so named because traditionally it has greater number of jurors than trial jury, called a petit jury (from the French word petit meaning small).
Although mice tend to come out in the night when most of the occupants of the house are asleep or immobile. This allows them to stealthily infiltrate any of your food bags, crumbs etc. During the day time, mice will normally not come out, but if opportunities exist then they surely can.
Again, this protects them from predators. But some animals don't just sleep at night; others sleep during the day. These animals may only come out at night! Some do this so that they don't get eaten by anything else out there during the day, because at night big predators are usually asleep. Others, such as bats, may only come out at night so that they don't have to share their food with others.