It's certainly not waterboarding or the like. But I too wouldn't like it if prison guards kept the lights on in my cell 24 hours a day.
Whether that constitutes "deliberate indifference" depends a lot on the facts; in particular, on just how bright it was. The facts here are exceptionally unclear. Which is why the Ninth Circuit remands.
Putting a tiny little night light into my cell to make sure I'm not fighting, or digging a hole, or doing other untoward things seems okay. Shining a spotlight on me 24 hours a day is a different matter.
I'm sure the present case is somewhere in between these two extremes. We'll find out precisely where on remand.