Actually Kendalle, I think you're thinking of some other creature. Guinea pigs are active both day and night. They nap at various times throughout both day and night, and then wake to eat and play. With the way their internal systems are designed, they would go into stasis if they laid down and slept all night. A common complaint from guinea pig owners that keep them in the bedroom is that they're very LOUD as they run laps and drink from their bottles and wheek to each other.
Even if the pig is only in the tiny cage at night, he's more than likely spending at least 12 hours by himself, bored out of his mind. If anyone locked a dog in a crate for 12 hours, people would not find that humane, so I'm not sure why it seems to be ok for piggies?
Also, guinea pigs should not be put together and separated for "playtimes" if at all possible, because it's extremely stressful for them. They sort out dominance when they meet with another pig, and will do so over and over and over again.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry