Stacking two SPF products does not create a stronger combined number.

Layering a moisturizer with SPF 20 under a foundation with SPF 15 does not give you SPF 35. It gives you SPF 20, and nothing more.

TODAY'S TOPIC: Skin and Hair

"Wearing a moisturizer with SPF and a separate sunscreen at the same time adds their protection together."

The instinct to add up SPF numbers makes intuitive sense, the same way stacking two blankets keeps you warmer. Sun protection factor simply does not work on that kind of math.

When you layer multiple products containing SPF, your final protection level equals the highest single SPF among the products, not the sum of all of them. An SPF 20 moisturizer under an SPF 30 sunscreen gives you SPF 30 protection, the same as the sunscreen alone, because the numbers describe UVB blocking percentage rather than a stackable quantity.

Layering can still help you cover missed spots and add a slight margin, but the number itself never grows past the strongest single product.

Where the evidence splits:

The advice for bad sleep arrives in a bottle, and when the bottle stops working the advice is to try melatonin again.

Here is the claim on the other side of that, and it comes with a number attached. A top sleep expert from the University of Washington puts a 30-second cherry trick at 10 times more effective than melatonin pills.

The number is measured on total sleep time. Studies are said to show the trick releases a “sleep-regulating” hormone, resets the sleep-wake cycle, and adds 84 minutes to how long you actually sleep — and it is that figure, not a general impression, that the 10X comparison rests on.

Not sleeping pills. Not cutting caffeine, breathing exercises, chamomile tea, or another doctor’s visit.

It is working like crazy for 36,498 people. Susan R., from Denver, said: “I finally started sleeping straight through. No more tossing and turning, no more groggy mornings. For the first time in years, I wake up feeling truly rested.”

Verdict: a claim with a named comparison and a measurable number behind it, which is more than the melatonin aisle offers. Whether those 84 minutes show up for you is the part only you can check.

>> See the 30-second cherry trick and the 84-minute claim behind it

"Hyaluronic acid serum can make skin drier rather than more hydrated in low-humidity environments."

Hyaluronic acid is marketed almost universally as a hydration powerhouse. In a specific set of conditions, the same ingredient can work against you.

Hyaluronic acid draws moisture toward itself, typically pulling it from the deeper layers of skin or the surrounding air. In very dry climates or low-humidity indoor environments, there may not be enough ambient moisture available, and the ingredient can end up pulling water from the skin's own deeper layers instead, potentially leaving skin drier than before application.

Sealing hyaluronic acid under a moisturizer with actual oils or emollients generally prevents this. Applied alone in dry air, the ingredient can behave differently than the marketing suggests.

The research:

"A simple at-home test, like watching whether a strand of hair floats or sinks in water, reliably determines your hair's porosity type."

This test has spread widely through haircare communities as a quick, at-home diagnostic. It turns out to be far less reliable than its popularity suggests.

The float test is affected heavily by factors unrelated to actual hair porosity, including the presence of product buildup, oils on the strand, and even the specific hair follicle it came from, since porosity can vary across different areas of the same head. Hair professionals generally consider the test inconsistent enough that it should not be used as a reliable basis for choosing products.

A test that different strands from your own head can fail differently is not measuring what people think it measures.

The research:

Some of the most repeated skincare and haircare shortcuts do not hold up once you look closely at the actual mechanism. Worth checking before building a routine around them.