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.
But this research was done on mice, not humans, and the reason for the weight gain is unknown. And a single mouse study should not cause us to toss out the wealth of evidence supporting the calorie in/calorie out theory. Still, there are good reasons to be cautious about eating at night.