Yes! Vitamin C + Pycnogenol is a powerful antioxidant and it neutralizes free radicals. Ultraviolet light emitted by the sun induces free radical formation in the skin and that leads to photoaging and even cancer. Vitamin C is a great antioxidant and it protects the skin against the harmful effects of UV radiation. So it’s not only safe to use this serum in the day, it in fact is helpful to do that. However, please note that this serum is not a replacement for your sunscreen and you should always layer your Vitamin C + Pycnogenol serum with a good sunscreen when using during the day.