That is false. As long as you only apply it before bed, at night...You are fine. Don't apply it during the day
Vitamin C is absolutely safe to use in Summer. It's probably one of the best ingredients you could be reaching for!
As an essential vitamin, It safely brightens and strengthens your skin, assists in collagen production and provides a bit of uv protection as well!
Vitamin C is a key component of collagen, the substance that binds cells together, and makes for healthy, vibrant, and youthful looking skin. Collagen relies on vitamin C for its stability, so that it can fight wrinkles, restore radiance, and even out the skin tone and texture. Vitamin C also functions as an antioxidant, protecting the skin from damaging free radicals. So, yes the serum would be fine in the summer. It is better to wear the oil at night because it gives the serum a chance to sink into your skin and start repairing.
Read more: http://www.ehow.com/way_5455463_time-use-vitamin-serum.html#ixzz2nTKPJTLV