Artifact is tracings caused by movement, either by the machine, leads, or patient- and means that nothing is really going on. He's probably doing the 24-hr EEG to see if there are more episodes of the sharp waves, and to see if you have symptoms that correlate with them. That's how my sz disorder was finally confirmed...the 24 hour EEG- most 20 minute EEGs are fairly useless unless its done in the middle of a seizure. The longer the reading, the better the information.
I had slow sharp waves on the right, near the frontal and temporal areas....was finally a relief that it was documented, so I could get treated- and not blown off, like I had been before- the 24 hour ones aren't bad- a little goofy looking, but it's only a day. I actually went to work with the leads on and the little pack that records things (it's about the size of a tape recorder or walkman- not that bad).
Artifact shows up in a lot of things, and the only way to get better info is to do a longer reading- probably get the best stuff when you're either asleep, or just doing your normal activities, rather than lying on a bed.