Are guinea Pigs nocturnal?

No. Guinea pigs are mostly active during the day, but will also get up, mooch about and eat at night. Don't we all :-) Hamsters are crepuscular, but guinea pigs are largely diurnal, although they can doze off for a few minutes with their eyes open on and off during the day. Sometimes guinea pigs will be asleep and then be startled if they are suddenly aware that you have come into the room.