It automatically turns itself off at night. And then back on again in the morning; and not in with the shocking "argh! my eyes!" suddenness of an incandescent, but a gradual taper that allows your eyes to adjust slowly.

Also, you could use a cup and a rubber-band to cover it if you needed darkness mid-day.

It's like a Window. You can “turn off” a window by putting a curtain over it.