From About.com:
"Mice are nocturnal and very social. They will be very active in the evening and night, but don't expect to do much with them during the day. Being social, they are best kept in groups. A pair of females is the easiest, although larger groups are fine if you provide the cage space. Pairs of males should be avoided, unless they are litter mates, never separated, and given a large enough cage that they can have their own space (unfamiliar males are very likely to fight). Keeping males and females together should be avoided unless you want
lots of mice in a short amount of time."
Henry was the only mouse at the shop the day I got him, so we can't get a litter mate for him. They have three or four now in a cage together, but who knows whether they're any relation to Henry. I'll just make sure he has lots of toys and spend as much time with him as possible so he doesn't get bored and lonesome. At first he was completely nocturnal and in the last couple of days, he's been taking naps and getting up to buzz around and play on his wheel at all hours. He was up when I got up this morning and having his breakfast. Or maybe for him it was supper.

But he's just as likely to be awake and active during the day.
He's a very neat little mouse. I've cleaned his aquarium twice since Saturday and it really wasn't absolutely necessary either time. It doesn't smell and even the newspaper he tore up to furnish his little house with is torn very neatly. I should've named him Felix Unger. LOL He does turn his food bowl over and spread out the food but I think he's picking through the mixture for the things he especially likes and rejecting the rest. I found some things hidden under his newspaper when I cleaned last night. I didn't even have to take him out to clean. He got into his little house and I just changed his newspaper and refilled his water bottle and wiped up a bit. But I figured when I clean bird cages every night I'll check to see if he needs maid service, too.