Obviously, cosmetic companies have weighed in on the side of the arsenal builders, encouraging women to use a different product for daytime care than they do for nighttime.
During the day, skin is subject to damaging UV rays, pollution, environmental stresses, and makeup, so creams designed for use during the day are primarily focused on protection and support. They contain SPF to prevent burning and photoaging, antioxidants to fight free radicals, and ingredients like caffeine to make skin look energized and firm. Day creams are usually designed to be worn under cosmetics, so they have light, nongreasy formulas that are absorbed quickly and allow pores to breathe. They provide a smooth, oil-free canvas for foundation to glide on smoothly.
Although some boast of anti-aging properties, daytime moisturizers are often light on compounds like alpha-hydroxy acids and retinoids because these compounds have a tendency to make skin more sensitive to the sun. Another reason is that the addition of too many active ingredients in one product can sometimes reduce their effectiveness. ($36, Boscia Oil-free Daily Hydration SPF 15 ; $31.50, La Roche-Posay Anthelios SX Daily Moisturizer )
Nighttime is when skin does its heavy lifting. Like the rest of the body, skin does the bulk of its repairing, restoring, and regenerating while we sleep, so night creams are focused on moisture and recovery. They contain the most powerful, slow-absorbing moisturizers that are designed to penetrate over the course of several hours; since there’s little to no concern about sun exposure, they also contain the highest concentrations of anti-aging compounds ingredients like retinol, glycolic acid, and hyaluronic acid, which are able to do their work without interference from sunscreens.
The result is that night creams are often richer, thicker, or heavier than day creams. Of course, we don’t mind much, since most of us don’t wear makeup to bed, and the cream is usually absorbed by the time we wake up. ($93, Shiseido Benefiance NutriPerfect Night Cream ; $37.50, Origins High Potency Night-A-Mins )
Why Use Both?
There is a real difference—purposewise and texturewise—between night creams and day creams. Will using a night cream in the morning cause a calamity? No. The worst that could happen is that your skin will feel greasier than normal and won’t be protected from the sun. Likewise, a day cream used at night will still provide some moisture and support, but your skin probably won’t get a full range of anti-aging and moisturizing ingredients.
The decision of whether to use two moisturizers or one is ultimately a personal choice. Some women choose to have two different products (plus eye creams and serums) to keep their skin in its best shape, while some find that one all-purpose moisturizer with moderate amounts of anti-aging ingredients, moisturizers, and sun protection works just fine. Creams like Clinique Moisture Surge Extended Thirst Relief  ($34) or Philosophy Hope in a Jar  ($42) can be great choices for young, healthy skin or for women with sensitive skin who want to use the minimum number of products.
So choose one moisturizer, choose two, or choose a whole cupboard full of them. As long as you’re protected from the sun during the day, it’s up to you to decide what fits your skin and your pocketbook.