Actually, the slot canyons are best seen near local noon as that's the time of day when the sun is most likely to make it into the canyon and light things up. As you move about in the canyons one area will light up then a bit later another and so it goes for a few hours.
So, DON'T stop for lunch -- be there for lunch -- eat before or after.
Lastly, the upper canyon has to be booked, usually the day before, from the offices in Page whereas the lower canyon can be driven up to and usually is less of a wait to get into. However, if you go there during the busy season, say May through September, you may have to wait a day or two.
Oh yeah, one more last thing -- it is VERY dusty and dirty in the canyons so be preparred to clean your equipment when you leave and use a lens that WON'T need to be changed inside. Think WA zoom! When I shot with my D100 a couple years ago I used the Sigma 15-30mm zoom and it was a perfect fit for this application. I know of some photographers that wrap there gear in plastic to protect it from the sandy/dusty/dirty crud in the air most of the time.
Brian