[lukas099]

> So, there must be some really good evolutionary reason for sleep.

Just one, but partitioning activities in time allows for further specialization. For example, instead of having generalist eyeballs that are okay at seeing in light and dark, you can have specialist ones that are really good at one or the other. Then another species in the same area that utilizes similar resources can specialize in the opposite, reducing competition between the two species (this is called resource partitioning).

Then, once there's a chunk of the day that is less productive than the other, it makes more sense for the body to devote that time to various recovery mechanisms. Then that chunk of time is even less productive, making using that time for recovery even more attractive.