Guinea pigs are most active at night. That’s when they do most of their eating and exercising.
This doesn’t mean they sleep all day, though. They will take long naps during the day, but will wake up and eat and interact with you or their cage mates for a while before going back to sleep.
The best time to handle and play with guinea pigs is in the evening. This is when they are most alert and receptive to interacting. You can also handle guinea pigs during the day when you see they are awake, although they may be less interactive at this time.