r and spfs
SPF can be a bit tricky, especially in acne based products since sometimes sensitive skin is associated with acne and not all SPF is mineral based. There's chemical based and mineral based (like natural zinc).
You can always buy a facial sunscreen or SPF product and mix it in with your lotion. Neutrogena has a fab liquid sunscreen that's super light weight and not greasy feeling. La Roche Posay is also another great brand and they have mineral based sunscreens.
SPF in as mild as a 20 or 30 can work wonders on the skin in protecting against oxidative damage and premature aging. Suncare can help prevent against the premature break down of collagen and elastin in skin, preserve an even skin tone, and obviously protect against the harmful rays that can lead to skin cancer.
The presence of SPF is to block UV rays from the sun, and supposedly having them in night creams dim the effectiveness cuz the good stuff don't sink in as well or something. In any case, SPF is important during the day and any website or Google search will tell you that. By blocking UV rays, SPF slows aging or wrinkles and help prevent cancer and bunch of other stuff. There's also chemical and physical SPF but that's for another discussion.
Everyone should use SPF during the day, and that's a fact. The amount of SPF differs by individual. I have light/med complexion and SPF15 is enough for me even in Texas so that I tan but don't burn. I do have friends who are so white they are more pink than white, and if they use anything less than SPF30, they burn like tomatoes. If you are very happy with your current moisturizer, you can use a lightweight/sheer sunscreen over it, or find the equivalent with SPF. Nowadays, almost all skincare lines have SPF moisturizer as well as non-SPF.
As for applications, treatment (peels, pads, serums, night cream with 100 good for you ingredients) should be done at night because that's when your body repairs itself and stuff are most effective. Some peels or serum include ingredients (Vit C and some anti-aging ones) that degrades in the sun or make the skin extra sun-sensitive, thus should be used at night and might be either ineffective or counterproductive during the day. During the day, SPF is of the most importance, so you can either use an SPF moisturizer, or your normal moisturizer with SPF over it.
Answered[ Edited ]
Okay, I am going to clear up a belief about SPF in skincare. Just because your skincare/makeup has SPF in it DOES NOT mean that you shouldn't wear another layer of sunscreen. There are a lot of times in which the SPF in skincare/makeup is so muted, that it is hardly effective in blocking out the UVA/UVB rays. Best thing to do is to wear additional sunscreen that is broad spectrum. UVA causes aging, such as dark spots and wrinkles, while UVB causes burning.
Hopefully, this will help you out a bit.