Now interval training is taking over your beauty regimen.

More than half of U.K. women believe that their skincare products stop working over time – the way your fitness plateaus if you stick to the same workout. So Avon Cosmetics has come up with a rotational anti-aging night cream – and it’s already got a 30,000-person waiting list ahead of its Sept. 9 U.K. release.

The $36 Anew Reversalist Infinite Effects Night Treatment Cream comes in a 2-in-1 tube that features two treatment formulas, which you rotate week-to-week. The first made with phytol – a chemical derived from plants – is used for seven days to prep your skin to receive the active anti-aging ingredient the following week – a retinol formula that supposedly reduces fine lines, wrinkles and brown spots. 

“By switching back and forth between these two creams every seven days your skin will reap the superior benefits of the anti-aging Super Retinol Complex,” the press release claims. “It’s the continuation of the rotational technology that helps provide continuous results that do not stall even over a full year.”

Avon’s own year-long clinical study of 116 women showed “significant results,” although Avon doesn’t clarify what exactly those results are. The beauty company has yet to respond to Moneyish requests for comment.

Avon isn’t the only one with skin in the interval treatment game. Lancome’s $75 Visionnaire Crescendo Dual-Phase Night Peel also has a two-phase approach over 28 days, which it describes as a “personal trainer for your skin.” Phase 1 calls for using a lighter formula of fruit acids and quinoa husk to shrink the appearance of pores and improve your skin’s texture for two weeks, followed by Phase 2’s stronger derma-acids to reduce the axis of skin evils: fine lines, wrinkles and dark spots.

It sounds good on paper, but Dr. Michele Green, a dermatologist at Lenox Hill Hospital in Manhattan, told Moneyish that while the retinol and phytol highlighted in Avon’s anticipated product are legit, she’s not sold on the idea of interval training your skin.

“Avon’s Anew products have shown some excellent results, but it’s not clear to me what this ‘super retinol complex’ is, or why you’d want to go on and off of it,” she said.

In fact, retinol is more effective when you use it continuously. “You want that accumulation, you want your skin to get used to it,” said Dr. Green. “One of the biggest side effects of going off the retinol or Retin-A is actually breakouts. So going on and off of it … I don’t get it.”

Dr. Jody A. Levine, director of dermatology at Plastic Surgery & Dermatology of NYC, agrees. “I don’t agree with this concept of your skin ‘getting used to things,'” she told Moneyish, noting that rosacea (a red-faced, acne-like skin disorder) is on the rise in women because they are trying so many different products, and sensitizing their skin in the process.

“I tell my patients to find a skin care regimen and stick to it, and not to rotate or expose themselves to too many ingredients,” Dr. Levine adds.

And Lancome’s concept of going from a less intensive peel to a stronger peel makes sense, Dr. Levine says – but then why would you go back to the weaker peel? “When I treat patients with severe acne, I have them come in for a series of six peels 10 to 14 days apart, and by the end I work them up to a strong peel that won’t leave them all red,” she adds. “You don’t go back down.”

And both derms agree with using retinol daily with moisturizer.

If a product appears to become ineffective, it could be that your skin is what’s changing: It’s getting older and changing texture, so you might need something stronger.

There’s a similar misconception among consumers that shampoos and conditioners “stop working” after a period of time, which is why many shoppers rotate formulas to keep their hair looking healthy and shiny.

Again, your brand probably isn’t to blame. Rather, product build-up on your scalp could be making your hair suddenly limp, greasy or unmanageable. Or environmental changes – like the air becoming drier or more humid – or hormonal changes in your own body, are what’s really giving you those bad hair days.

It could also be a matter of whether you’re really using the product correctly and consistently. A Procter & Gamble study into whether dandruff shampoo becomes less effective after three months – as the subjects believed – found that scalp treatments of different strengths all worked consistently over the course of an entire year. Researchers suggested “compliance” could be the culprit; are users still following the instructions and using the correct amount months later?

It could also just be in your head. “You might need to switch products to treat the new condition of your skin or hair … but the other product didn’t just ‘stop working,’” said Dr. Green. “Or when you get tired of using something, you start looking for reasons to use something new.”