Keeping a consistent skincare routine is one of the best ways to boost skin health — but does applying certain ingredients at night really make a difference?

Spoiler alert: it does. Experts agree that when you apply your skincare products can have a dramatic impact on their effectiveness. This is due to something called circadian rhythm. We’ll tell you what that means for your skin — and walk you through what works best before bed.

What is circadian rhythm?

You may have heard the term circadian rhythm: it’s the body’s 24-hour internal “clock” that regulates our metabolism, physiology (cellular function) and behavior. This biological clock is an incredibly complex system that’s coordinated by a cluster of nerve cells called the suprachiasmatic nucleus (SCN), located within the brain’s hypothalamus.

The SCN is responsible for sending messages that stimulate genes to produce proteins that in turn, tell every cell in your body what to do. This affects a variety of essential functions, like:

• Sleep patterns
• Hormone release
• Eating habits and digestion
• Body temperature

How does circadian rhythm affect skin?

Organs like your skin are regulated by your body’s master clock, but they also have their own circadian rhythms. Genetic programs regulate the release of a variety of chemicals, like hormones and enzymes, that send messages and stimulate reactions within skin cells.

Your body's biological clock determines a host of skin processes and characteristics, including:

• Temperature
• Blood flow
• pH
• Permeability
• Cell division
• Damage repair

What happens to your skin at night?

Until recently, most of the recommendations about which products to use at nighttime came from anecdotal evidence: dermatologists, patients, and skincare professionals noticing that some skincare seemed to perform better when applied in the evening. But recent studies have quantified those beliefs, shedding light on how and why this happens. Here’s a roundup of how your biological clock impacts your skin overnight.

Skin barrier permeability

Studies show that the stratum corneum is more permeable at nighttime. Though more research is needed, this is likely due to a combination of chemical signals and a decrease in sebum levels. This permeability has two significant implications:

• Transepidermal water loss (TEWL): essentially, your skin’s more likely to lose water overnight. This is why experts typically recommend using moisturizer after nightly cleansing. SLMD Facial Moisturizer is a lightweight lotion that’s great for all skin types (including oily or acne prone). SLMD Hyaluronic Acid Serum also contains squalane, so it deeply hydrates and locks in moisture too.
• Increased active ingredient absorption: simply put, products sink into your skin better at night. This makes it an ideal time to apply actives like retinol. SLMD Retinol Resurfacing Serum is time-released, which helps it work gently throughout the night.

Skin cell proliferation

While different cell types in the body all follow their own unique rhythm, experts generally agree that skin cell division peaks during the nighttime hours. This is another reason why it’s ideal to use retinol during your evening routine, because it stimulates the cell turnover process.

Some of the most innovative anti-aging retinol products are formulated with skin nourishing ingredients, to strengthen the skin barrier while boosting cell turnover at the same time. SLMD Retinol Resurfacing Serum contains both hyaluronic acid to plump and vitamin C to help repair free radical damage.

Melatonin increases

You’ve probably heard of melatonin, which is a hormone that increases in the evening, creating a cascade of biological processes. Studies show that melatonin affects your skin in several ways:

• Stimulates hair growth
• Repairs UV damage
• Accelerates wound healing

Human growth hormone increases

Studies have also demonstrated an increase in nighttime human growth hormone (hGH) levels. Though more research is needed, some data indicates this may help prevent signs of premature aging, like collagen loss.

Cortisol levels drop

Also known as the stress hormone, cortisol (in healthy amounts) is crucial to a variety of functions, from boosting energy to reducing inflammation. Levels typically dip before bedtime, which can impact your skin.

Ever notice how insect bites or a patch of eczema itches more when you’re trying to fall asleep? That’s not just because you’re less distracted: it’s because there’s less cortisol to calm your immune response. Dr. Lee recommends keeping an over-the-counter hydrocortisone cream in your cabinet to help soothe stings, bites, rashes and dermatitis.