I realize that there is some overlap but for the main characters of CSI being part of the "graveyard shift" it seems like they work a lot during the day. Any idea why this is?
According to the CSI Wiki:
This doesn't explain why they're seen during the day, but just gives an outline of the time patterns of each shift.
The most common explanations I've seen is that when they start a case, they are determined to complete it as quickly as possible and thus stay well beyond their normal hours (not that uncommon in cop dramas in particular). You can even hear the odd uttering by one of the team that they're "working a double" in some episodes. Finally, the faster they work, the more likely they are to secure a conviction (before evidence is destroyed/lost and the culprit moves on/covers their tracks, etc). All of these reasons provide suggestions as to why they are often seen working well beyond their normal hours.
These are all obviously just my opinions, but I don't think there is an official line on this apparent discrepancy.