Wearable fitness trackers, smart rings and watches are commonplace, and now so is knowing details about your own body’s inner workings. One metric, once niche, has gone mainstream: heart rate variability, or HRV. Millions of people now wake up to daily HRV scores, alerts, and color-coded judgments about stress and recovery.

How it works: Heart rate variability measures the tiny differences in time between consecutive heartbeats, usually in milliseconds. Those fluctuations reflect how the autonomic nervous system balances “fight or flight” responses with “rest and digest,” offering insight into how well the body adapts to stress and recovers. Anywhere from roughly 20 to 70 milliseconds is considered within normal range.

Real-world application: A high HRV (closer to 70) indicates the body is adapting to stressors and can recover more quickly. It’s a sign of a balanced autonomic nervous system and a higher level of cardiovascular fitness. Low HRV signals the opposite—that the body is less able to adjust to the ups and downs of life. Stress, anxiety, high blood pressure, inadequate sleep, dehydration and new medicines are among the many things that can lower HRV.

What the experts say: What’s more important for the average person, experts say, is the relative change over time. “HRV is most powerful when you’re measuring it over several weeks and can see a graphic trend on how it’s being affected by everything that’s going on in your life,” says Bryan Wilner, an electrophysiologist at the Baptist Health Miami Cardiac and Vascular Institute. Drinking more water and doing strenuous exercise are two ways to improve HRV.

For many species of reptiles, whether their offspring develop into males or females is dictated by temperature. For example, for green sea turtles, if the temperature of the sand where mothers bury their eggs is about 29 degrees Celsius (84 degrees Fahrenheit) during a critical mid-incubation window, the babies will hatch as a half-and-half mix of females and males. But the hotter the nest, the more they will skew female. The opposite is true for the American alligator: If their eggs incubate at a little below 32 degrees C, they’ll hatch as an even mix of males and females. If they incubate above this temperature, more hatchlings will be male (if the temperature gets hot enough, the ratio will skew back toward females). Scientists have predicted nearly single-sex generations of alligators by the year 2100.

Why this matters: If rising global temperatures mean entire generations of sexually reproducing reptiles will be dramatically skewed male or female, such imbalances could doom species, writes Elizabeth Preston: mating opportunities will decline and populations might become inbred as surviving members of species struggle to find mates.

What the experts say: Some species may be capable of adapting their nesting habits to account for warmer temperatures–one study on wild painted sea turtles from different climates found that the mothers adjusted where they bury their eggs depending on current conditions. But altering nesting behaviors assumes that all species have enough physical spaces in which to do that, which isn’t the case. Many species are enduring drastic habitat loss from human activities. In our human-dominated world, says Benjamin Parrott, an ecologist at the University of Georgia, migration might not be a feasible solution. “I don’t think people in D.C. are going to tolerate gators in the Potomac,” he says.