Thus far, we’ve looked at why getting some sun is actually good for you, how to wisely choose and apply sunblock, and how to time your sun exposure to allow the maximum number of hours outside.

Today, however, we’ll be working from the inside out, starting with Polypodium leucotomos, a green leafy fern found in the wilds of Central and South America. Polypodium initially evolved as an aquatic plant, before a changing environment forced it to adapt to life on land. Unable to leverage the sun-blocking effects of water-cover as it had when it lived underwater, the fern instead evolved to produce powerful antioxidants that offset the free-radical damage of all-day above-water sun.

As recent research has shown, those fern antioxidants work nearly as well inside of you, too. By taking pills that contain Polypodium leucotomos extracts, like Heliocare or Solaricare, you can triple or quadruple your natural resistance to burns. In other words, if it might normally take you 15-20 minutes to scorch at a given UV intensity, you could instead hold out for a full hour.

And while, in most cases, that’s not enough to supplant sunscreen, given how quickly sunscreen sweats and washes off (as previously discussed, ‘waterproof’ sunscreens are designed to weather just 40 minutes of swimming and sweating), a belt-and-suspenders approach seems like reasonable insurance.

Pick up some Heliocare or Solaricare, pop one in the morning, and another before and every few hours during your time in the sun. If nothing else, you can offset the $20 cost of a bottle by the money you’ll save on aloe vera. (Which, as we’ll see in the next installment, doesn’t really do much of anything anyway.)

Researchers who follow hunter-gatherer tribes in tropical and dessert areas have found a nearly universal pattern: during the very hottest hour or two of the day, the members of the tribe get out of the sun completely, to relax and eat in the shade.

Over the course of a summer day, the UV index – the amount of UV radiation reaching ground level – varies hugely. At 12:30pm today in New York, for example, the UV index was at 10, enough to cause burns in just 10 minutes, blazing through even strong sunscreen. Whereas by 1:15, the index had dropped to a 6, allowing for a half hour before burning without protection, and for several hours of happy sun time with a layer of (full-spectrum) sunscreen applied.

A team of outdoorsy engineers in New Zealand recently released a free app, UV Lens, which provides daily hyper-local UV forecasts. With the app in hand, you can easily plan your schedule to mimic the wisdom of our hunter-gatherer ancestors: enjoy the sun in the morning, take a brief, strategically timed mid-day lunch break in the shade, and then head back out once the very highest UV stretch of the day has passed. That way, you can spend far longer outside overall, while still greatly reducing the risk of sun-damage and burn over the course of the day.

As I shared in Part I, getting some sun is good for you, at least if you’re smart and careful about it.

Your first step to that end: get some good sunscreen.

Sunlight is made up of two kinds of ultraviolet rays: UVA and UVB. It’s the latter, UVB, that causes sunburns, so for decades sunscreen was designed to block UVB. But more recently, research has shown that UVA rays, which penetrate deeper, also substantially increase skin cancer risk, and cause wrinkles. (The EPA estimates that up to 90% of aging-related skin changes are actually caused by a lifetime’s exposure to UVA.)

Good sunscreen is therefore ‘full spectrum’ or ‘broad spectrum’, and blocks both UVA and UVB. While those were previously specialty products, in the last year or two, almost all the major sunscreen brands have released reasonably-priced, widely-available versions that block both spectrums. Make sure you only buy sunscreens that do.

Three more sunscreen tips: slather it on, do it often, and stop going nuts with the SPF.

According to the American Academy of Dermatology, most people apply only 25-50% of the amount of sunscreen that they need per application, which reduces an SPF 30 sunscreen to an SPF 3. SPF ratings are based on applying two milligrams of sunscreen per square centimeter of skin. Which is a lot. Basically, you should briefly look like Casper each time you apply a layer if you want your sunscreen to actually do anything.

Next, a sunscreen is FDA-certified as ‘water resistant’ if it can hold up to 40 minutes of swimming or sweating. After that, all bets are off. So, while you’re on the beach, you also probably need to reapply every hour or so.

And, finally, just buy some SPF 30; after that, the numbers get kind of meaningless. A few years back, Procter & Gamble even sent a letter to the FDA, asking that the numbers be capped at 30, because real-world and laboratory light conditions are different enough to make higher SPFs of “dubious value” that are “at best, misleading to consumers.”

So, to recap, buy some SPF30 full-spectrum, water-resistant sunscreen. Put on a bunch, and keep reapplying. And then enjoy the sun!

With beach weather upon us, I’m spending this week on a roundup of summer sun tips, with the science behind each, so you can make smarter choices about what works, and what doesn’t.

First up: get some sun. It’s good for you.

While people completely ignore most public health advice, it seems we’ve actually taken warnings about the dangers of tanning too much heart.

Excess sun exposure (and sunburn) increases the risk of skin cancer. But too little sun exposure dangerously decreases your level of vitamin D (which your skin naturally produces when exposed to sun), increasing the risk of a slew of other cancers and heart disease.

As one recent review study concluded, “the overall health benefit of an improved vitamin D status may be more important than the possibly increased melanoma risk resulting from carefully increasing UV exposure.”

In other words, it’s healthy to get back out in the sun. Just be smart and careful about how you do it. Tune in tomorrow, and learn how to wear (good) sunscreen, the right way.