Yep, Brad is very correct about the activity levels during the day . . .
I was lucky enough to go on an actual African Safari last summer (their winter.) The lions, for the most part, sat around during the day and were active at night. It was cooler when we were there during the day, but I don't even think it has a lot to do with heat. They hunt and are most active at night. That is what they are programed to do. They conserve energy during the day and roam around at night.