Things your phone probably reminds you of: Your mother’s birthday. Your kid’s doctor appointment. That thing Beyoncé said on Twitter.

Now, thanks to scientists at Northwestern University’s Center on Bio-Integrated Electronics, your phone can also tell you exactly how much sunlight your body has absorbed that day, based on what you’re wearing, what the weather is and where you are located on the globe.

This useful info comes courtesy of a tiny sensor developed by researchers John A. Rogers and Dr. Steve Xu that can stick to your skin or clip to your hat. “It’s smaller than a dime, thinner than a credit card,” Xu said, “and you can stick it or clip it anywhere, which allows people to customize it.”

His favorite application is using it as nail art. (Scientists love the fingernail as a vehicle for a wearable device, he said, because it’s stable and durable.)

The sensor is so small, Xu said, “I often forget I’m wearing it.” Yet, the device packs a lot of power and data-gathering ability: It can accurately measure UVA and UVB radiation, as well as light exposure, runs on solar power without a battery and never needs recharging.

Getting rid of the need to charge not only makes it easier to use, Xu said, “it allows the device to be even smaller, and cheaper to make.” It’s also virtually indestructible — students dropped it into boiling water and simulated running it through a washing machine but were not able to break it.

The accompanying phone app allows users to enter information about sunscreen applied, clothing and activities (such as whether you’re in the water.) “It’s really a platform technology,” Xu said, “that can measure light extremely accurately in a novel way.”

That’s important, he said, because sun exposure is the No. 1 contributor to skin cancer.

“One in five Americans will have skin cancer in their lifetimes, and that’s really pretty scary. But if you think about when we’re outside enjoying ourselves, we are just guessing about how much sun exposure we are getting.” And every sunburn increases the chance of skin cancer. “All of that,” Xu said, “translates to an increased lifetime risk.”

The need for better protection from UV radiation is why there is a consumer version of the sensor, called “My Skin Track UV,” that was developed with L’Oreal. It launched in November at the Apple store.

Xu said the device’s next version has other applications that dermatologists like him are excited about, such as the ability to allow doctors to carefully track sun exposure for skin cancer survivors.

“Light is one of the world’s oldest medicines,” he said, “and we use it to treat diseases.” These include skin diseases, seasonal affective disorder and jaundice in infants. The new sensor is able to accurately measure light exposure that patients are getting from light therapy, so that it can be adjusted for greatest benefit.