In a photo posted to Instagram earlier this year, Dave Asprey, founder and CEO of Bulletproof Coffee, smiles big as he flexes his left arm, revealing a small white disk attached to this tricep.

"It's official. I'm a cyborg," the caption reads.

In Silicon Valley, a growing number of entrepreneurs and biohackers are using a medical technology called a continuous glucose monitor, or CGM, in order to learn more about how their bodies work. They wear the device under their skin for weeks at a time.

CGMs, which cropped up on the market less than 10 years ago and became popular in the last few years, are typically prescribed by doctors to patients living with diabetes (both types 1 and 2). They test glucose level, or the amount of sugar in a person's blood, and send real-time results to a phone or tablet. Unlike fingerstick tests, CGMs collect data passively, painlessly, and often.

For people taking a DIY approach to biology, CGMs offer a way to quantify the results of at-home experiments around fasting, exercise, stress, and sleep.

A patient checks their blood glucose level using a conventional fingerstick test. Stoyan Nenov/Reuters

Asprey, a self-proclaimed biohacker who's spent over $1 million on "smart drugs," wearable tracking devices, and diagnostics tests in the pursuit of cognitive enhancement, has been wearing a CGM on and off for two months. He does not have diabetes, though he's familiar with fingerstick tests from 20 years ago when he weighed 300 pounds and was told by his doctor to monitor his glucose.

In 2017, Asprey bought the device from a European vendor online. (In the US, patients need a prescription to get a CGM.) He's has been wearing it during a cross-country tour for his new book, "Head Strong," so he can track how sleep disruption due to jet lag affects glucose.

A small body of research suggests that a lack of sleep may contribute to the development of type 2 diabetes, the most common form of the disease. In 2016, Dr. Frank B. Hu, a professor of nutrition and epidemiology at Harvard, published the results of a study that followed over 133,000 (generally healthy) women for 10 years. The researchers found that women who self-reported difficulty sleeping were associated with a 25% increased risk for type 2 diabetes.

"Wearing one of these all of the time would be kind of absurd," Asprey says, touching the area around his implant. "I wanted to see what would happen."

Asprey found that his glucose level is normal, and his standard breakfast of Bulletproof Coffee — a proprietary blend of grass-fed butter, oil, and coffee — did not cause an increase in glucose. (Asprey's experience is subjective and does not suggest the beverage is safe to drink for all.)

His studies on the relationship between sleep deprivation and glucose are ongoing, but Asprey doesn't plan to wear the device long-term because it's invasive and his health doesn't require it.

Paul Benigeri, a lead engineer at Nootrobox, breathes a sigh of relief after a continuous glucose monitor is painlessly implanted under his skin. Melia Robinson/Business Insider

Few entrepreneurs who wear CGMs actually live with diabetes.

At Nootrobox, a startup that makes cognitive enhancement supplements, or "smart drugs," a majority of the company's dozen employees subscribe to an increasingly popular diet called intermittent fasting, which involves going without food for anywhere from hours to several days. They wear CGMs so they can see how the food they eat spikes their glucose in real time.

Kevin Rose, the cofounder of Digg and a serial entrepreneur, was prescribed and wore a CGM for a brief time. Like the employees of Nootrobox, he got one for the love of data.

"I'm somewhat a little bit of a 'body hacker.' I like to try out different things to see how it impacts my overall wellbeing," Rose told Business Insider in January. "I put [the CGM] on and I wore it for about a month. When I did, I would throw everything at it."

He drank a glass of pulp-less juice just to watch his glucose level go "through the roof."

Patrick Collison, CEO and cofounder of mobile payments startup Stripe, said on Twitter in 2016 that he bought a FreeStyle Libre Flash Glucose Monitoring System in Europe. Collison does not live with diabetes, he said, but has become interested in glucose monitoring since learning how different foods affect insulin response, which is regulated by blood sugar levels.

Chrissy Ensley, head of operations and supply chain at Nootrobox, checks her blood sugar levels regularly after installing her continuous glucose monitor. Melia Robinson/Business Insider

The early adopters believe if they can access the data around their own glucose levels and take action to lead healthier lives now, their efforts may help them stave off obesity, diabetes, and other life-threatening diseases in the future. They expect CGMs to become more common.

Asprey, of Bulletproof, says the CGM has taught him what low blood sugar and high blood sugar feels like. He hopes to apply those learnings toward making better lifestyle choices.

"The whole point of monitoring isn't to rely on the monitoring," Asprey says. It's about having the awareness to make confident decisions about the body you live in.