[erdevs]

Many nocturnal animals have eyes and utilize vision of light. Also, many animals who wake during the day and utilize vision of light also frequently hunt at night (eg see lions during hotter seasons).

It may well be that specializing systems such that they perform well during daylight only could lead to greater advantage than maintaining additional mechanisms for performing well at night also (or vice versa), and that sleeping during the downtime in order to conserve energy might confer supplemental advantage in this scheme. But we don't know if this explains all sleep, in all species.

Even if we did, then sleep in animals which perform reasonably well at any time of day would still be unexplained. Common ancestry wherein sleep evolved earlier may explain that, but we don't have that evidence as far as I know.

As such, it doesn't seem this is quite so easy to conclude about, let alone obviously so.