If you're like many people, you may have decided that you want to spend less time staring at your phone.
It's a good idea. An increasing body of evidence suggests that the time we spend on our smartphones is interfering with our sleep, self-esteem, relationships, memory, attention spans, creativity, productivity and problem-solving and decision-making skills.
But there is another reason for us to rethink our relationships with our devices. By chronically raising levels of cortisol, the body's main stress hormone, our phones may be threatening our health and shortening our lives.
Until now, most discussions of phones' biochemical effects have focused on dopamine, a brain chemical that helps us form habits - and addictions. Like slot machines, smartphones and apps are explicitly designed to trigger dopamine's release, with the goal of making our devices difficult to put down.
This manipulation of our dopamine systems is why many experts believe that we are developing behavioural addictions to our phones. But our phones' effects on cortisol are potentially even more alarming.
Cortisol is our primary fight-or-flight hormone. Its release triggers physiological changes, such as spikes in blood pressure, heart rate and blood sugar, that help us react to and survive acute physical threats.
These effects can be lifesaving if you are actually in physical danger - like, say, you're being charged by a bull.
But our bodies also release cortisol in response to emotional stressors where an increased heart rate isn't going to do much good, such as checking your phone to find an angry email from your boss.
Any time you check your phone, you're likely to find something else stressful waiting for you.
If they happened only occasionally, phone-induced cortisol spikes might not matter. But the average American spends four hours a day staring at their smartphone and keeps it within arm's reach nearly all the time, according to a tracking app called Moment.
The result, as Google has noted in a report, is that "mobile devices loaded with social media, email and news apps create "a constant sense of obligation, generating unintended personal stress".
"Your cortisol levels are elevated when your phone is in sight or nearby, or when you hear it or even think you hear it," said David Greenfield, professor of clinical psychiatry at the University of Connecticut School of Medicine and founder of the Centre for Internet and Technology Addiction. "It's a stress response, and it feels unpleasant, and the body's natural response is to want to check the phone to make the stress go away."
But while doing so might soothe you for a second, it probably will make things worse in the long run. Any time you check your phone, you're likely to find something else stressful waiting for you, leading to another spike in cortisol and another craving to check your phone to make your anxiety go away.
This cycle, when continuously reinforced, leads to chronically elevated cortisol levels.
And chronically elevated cortisol levels have been tied to an increased risk of serious health problems, including depression, obesity, metabolic syndrome, Type 2 diabetes, fertility issues, high blood pressure, heart attack, dementia and stroke.
"Every chronic disease we know of is exacerbated by stress," said Dr Robert Lustig, emeritus professor in pediatric endocrinology at the University of California, San Francisco, and author of The Hacking of the American Mind. And our phones are absolutely contributing to this."
In addition to its potential long-term health consequences, smartphone-induced stress affects us in more immediately life-threatening ways.
Elevated cortisol levels impair the prefrontal cortex, an area of the brain critical for decision-making and rational thought. "The prefrontal cortex is the brain's Jiminy Cricket," Lustig said. "It keeps us from doing stupid things."
Impairment of the prefrontal cortex decreases self-control. When coupled with a powerful desire to allay our anxiety, this can lead us to do things that may be stress-relieving in the moment but are potentially fatal, such as texting while driving.
The effects of stress can be amplified even further if we are constantly worrying that something bad is about to happen, whether it's a physical attack or an infuriating comment on social media. (In the case of phones, this state of hyper-vigilance sometimes manifests as "phantom vibrations," in which people feel their phone vibrating in their pocket when their phone isn't even there.)
"Everything that we do, everything we experience, can influence our physiology and change circuits in our brain in ways that make us more or less reactive to stress," said Bruce McEwen, head of the Harold and Margaret Milliken Hatch Laboratory of Neuroendocrinology at The Rockefeller University.
McEwen also noted that our baseline cortisol levels ebb and flow in a regular 24-hour cycle that is thrown out of whack if we get less than seven to eight hours of sleep a night, which is all too easy to do if you're in the habit of checking your phone before bed. This in turn leaves our bodies less resilient to stress and increases our risk of all the stress-related health conditions mentioned above.
Put this all together, and the hours we spend compulsively checking our phones may add up to much more than a waste of time.
The good news is that if we break this anxiety-driven cycle, we can reduce our cortisol levels, which in turn may both improve our short-term judgment and lower our risks for long-term stress-related health problems.
Over time, McEwen said, it's even possible to retrain our brains so that our stress responses are no longer on such a hair-trigger to begin with.
To make your phone less stressful, start by turning off all notifications except for the ones you actually want to receive.
Next, pay attention to how individual apps make you feel when you use them. Which do you check out of anxiety? Which leave you feeling stressed? Hide these apps in a folder off your home screen. Or, better yet, delete them for a few days and see how it feels.
And while you're at it, start paying attention to how individual apps affect you physically, too.
"If we're not aware of our physical sensations, we're not going to change our behaviours," said Dr Judson Brewer, director of research and innovation at the Mindfulness Centre at Brown University and author of The Craving Mind. According to Brewer, stress and anxiety often manifest as a feeling of contraction in the chest.
Regular breaks can also be an effective way to rebalance your body's chemistry and regain your sense of control.
A 24-hour "digital Sabbath" can be surprisingly soothing (once the initial twitchiness subsides), but even just leaving your phone behind when you get lunch is a step in the right direction.
Also, try to notice what anxiety-induced phone cravings feel like in your brain and body - without immediately giving in to them.
"If you practice noticing what is happening inside yourself, you will realise that you can choose how to respond," said Jack Kornfield, a Buddhist teacher at the Spirit Rock Meditation Centre in California. "We don't have to be at the mercy of algorithms that are promoting the fear of missing out."
Unfortunately, it isn't easy to create healthy boundaries with devices that are deliberately designed to discourage them. But by reducing our stress levels, doing so won't just make us feel better day-to-day. It might actually lengthen our lives.