There's no finite answer to this question. No studies have determined an ideal amount of time to wait in between each skincare product to get you maximum benefits, but there are a few products, like retinol and sunscreen, that are best applied with time in mind.
Product Penetration vs. Absorption:
Did you know that most products applied to the skin don't make it past the upper layers of the skin (which is mostly dead skin cells)? That's partly by design and partly because of skin anatomy. Our skin is quite good at keeping things out. That's why we have skin in the first place!
Penetration refers to products entering the skin's layers.
Absorption refers to products penetrating and eventually making their way into the bloodstream.
Factors that control product penetration on the skin:
- Heat and moisture: Certain products will penetrate deeper when there's moisture or heat on the skin.
- Skin integrity: If the skin barrier is damaged or weakened, ingredients can make their way into the skin more easily. This happens on purpose in some cases --like exfoliating to increase product penetration. In many cases though, a damaged skin barrier can result in irritation when certain ingredients enter the skin this way.
- Concentration: Skin penetration increases with a higher active ingredient concentrations, until a certain concentration is reached. After that, skin penetration remains constant even if higher levels are applied.
Oil soluble vs. water soluble: Oil soluble ingredients have a better chance of penetrating the outer layers of the skin because of the skin's water proofing mechanisms. Water soluble ingredients are more easily repelled which is why pH level and polarity (positive/negative charges) come into play during formulation.
Molecular size: Some molecules are just too big to penetrate into the deep layers of the skin (like waxes and heavy moisturizing ingredients).
Area of body: Skin thickness and the location of oil glands varies depending on the area of your body. The skin on the soles of your feet is very thick and has no oil glands or hair follicles, but the skin on your eyelids is much thinner. The thinner the skin, the easier it is for products to penetrate.
Time left on skin: Are you using a leave-on or wash-off product? Ingredients in a cleanser are less likely to penetrate into the skin than a serum that's left on the skin.
Ingredient: Some ingredients are better at penetrating and absorbing into the body than others. Hydroquinone for example has research showing a high systemic absorption rate.
Just because a product doesn't penetrate deeply, doesn't mean it's not working!
Some products, like mineral sunscreen or moisturizing creams, which contain a lot of emollients and occlusive ingredients and are meant to remain on the surface of the skin (the upper layers of your stratum corneum). This helps to soften the surface, as well as, protect against moisture loss and environmental pollutants. It also makes your skin feel nice!
Some of your moisturizers may be designed with active ingredients like vitamin c or retinol. Well-formulated products are designed to work with other ingredients to penetrate deeply in order to alter the skin's mechanisms and stimulate certain functions like collagen synthesis and repair.