The Murad rep told me the following: This can be used day or night, but it does not contain an SPF. So be sure that your foundation, tinted moisturizer, etc will help prevent sun damage.
From my personal experience, I would actually prefer something thicker for night use. I like the sheer texture of this for daytime wear, but again, no SPF.
Using it 2x a day for roughly 2 weeks has eliminated one of my larger (though not deep-set) wrinkles, though.
You can wear it at night! There is no SPF or any reason not to. I wear this in the morning and something more hydrating at night!