Why resort pools and beaches start to empty out after 5pm ?

I was in a Mexican resort last week with my wife and we started noticing that the pools and beaches were less crowded , almost empty around 6pm . We found this situation funny because this is the time that we decide to go out for our afternoon, because the weather is less hot and agreable. We also noticed at that moment that people start to pack all their stuff and go back to their room for their night supper .At 5pm ??? We come from a Mediterranean country , and normally 5pm it's the best moment to go at the beach for the afternoon after a good nap ( la siesta) during the hot hours (1pm to 4pm) . I don't wanna judge badly about this schedule but staying at the beach or pool, and having a tan during these hot hours is really dangerous for the skin .