Some domesticated rodents, like hamsters, can sink into a state of hibernation when exposed to too-cold temperatures. Mice aren't necessarily as sensitive to cold temperatures as hamsters, and they don't hibernate. However, their peculiar and sometimes unpredictable sleep habits may make it seem that way -- don't let their unusual schedules fool you.
Mice like to get their beauty rest, and a lot of it. A pet mouse can sleep about 12 hours a day, and while they have a reputation for being nocturnal, this isn't necessarily the case with domesticated pet mice. Yours may fall in and out of sleep sporadically or spend large chunks of time alternating between sleeping and being awake, so don't be alarmed if it seems like he's been asleep for way too long -- he's not hibernating.
While cooler temperatures won't send your mouse into hibernation like they might with other rodents, that doesn't mean he's impervious to the cold. If it gets too chilly, he won't hibernate, but he could get sick and/or succumb to freezing. A mouse's environment should be between 64 and 79 degrees Fahrenheit. Any lower, and he's not going to be able to adapt.
Do Not Disturb
If your mouse has been sleeping for what seems like a long time, don't be tempted to wake him up. Mice are very particular about their sleep, and if you disturb his slumber, you'll be greeted by a pet who is grumpy, at best. If you're really concerned, or waking him up is of some vital importance, use a cotton swab or the rubber eraser on a pencil to gently and slowly stroke him until he wakes up -- this way you won't get a bite.
Environmental Health Factors
A mouse's ability to withstand the cold is directly related to his natural resources. Remember, he won't go into hibernation, despite the fact that some critters like him would. But if a mouse doesn't have water at his disposal, for example, he won't be able to handle big fluctuations in temperature. Water helps him stay healthy and sleep on a normal schedule. Your mouse should be in an environment without exposure to wind or direct sunlight, and his water should always be filled and fresh.