Guinea pigs are not the most active creatures in the day. That can leave people thinking that they are nocturnal creatures, sleeping while you are out at work, shopping, or just general running around. It is easy to think that they are a lot more active when we are sleeping, as they seem to not do a lot at all, other than sit in their pen and make the occasional noise. However, they are not actually nocturnal or diurnal.
Guinea pigs are very habitual creatures, wanting to be fed at the same time each day, doing similar things at similar times, but always lying around not doing a lot of anything. While they are creatures of habit, they actually have a very scattered sleep pattern, sleeping whenever they want. However, they conserve energy most of the time because they are not very active, meaning they do not need a lot of sleep.
So, if you ever see your guinea pig sleeping, it is a relatively rare site that you should consider yourself lucky to see.