Night mode by itself uses more power than day mode, because the liquid crystal display has to work harder to display dark colors. However, there is an auto dimming feature that dims the led back light in dark conditions, even though you don't adjust the brightness. That explains why you get better battery at night. I see guys running night mode during the day all the time, and it cracks me up a little inside.

I only learned this when I was researching the advantages of the Samsung AMOLED screen over apple's LED back lit LCD displays.

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