NEW YORK • You are known to post online often. Suddenly, you stop. Does it mean you are depressed - or, who knows, you could have gone to a remote camping site?
Still, there is growing interest that your digital footprint - how quickly you scroll through your contacts, how frequently you check your phone at night - could hold clues to your physical and mental health.
The emerging field, called digital phenotyping, is trying to assess people's well-being based on their interactions with digital devices.
Researchers and technology firms are tracking users' social media posts, calls, scrolls and clicks in search of behavioural changes that could correlate with disease symptoms.
"Our interactions with the digital world could unlock secrets of disease," said Dr Sachin H. Jain, chief executive officer of health system CareMore Health, who has helped study Twitter posts for signs of sleep problems.
Similar approaches, he added, might some day help gauge whether patients' medicines are working.
The field is so new and so little studied, however, that even proponents warn that some digital phenotyping may be no better at detecting health problems than a crystal ball.
If a sociable person suddenly stopped texting friends, for instance, it might indicate that he had become depressed, said Dr Steve Steinhubl, director of digital medicine at the Scripps Translational Science Institute in San Diego.
Or "it could mean that somebody's just going on a camping trip and has changed his normal behaviour".
Still, the possible conflicting signals are not stopping the rush into the field by start-ups and technology giants.
Facebook recently announced that it was using artificial intelligence to scan posts and live video streams for signs of possible suicidal thoughts.
If the system detects certain language patterns - such as friends posting comments such "Can I help?" or "Are you OK?" - it may alert its review team.
In some cases, Facebook sends users a supportive notice with suggestions like "Call a helpline".
In urgent cases, it has worked with the authorities to dispatch help to the user's location. The company said, over a month, its response team had worked with emergency workers more than 100 times.
"It's a great idea and a huge unmet need," Dr Steinhubl said. Even so, he added, Facebook is "certainly right up to that line of practising medicine not only without a licence, but also maybe without proof that what they are doing provides more benefit that harm".
Professor Frank Pasquale, a law professor at the University of Maryland, wondered if "once you are characterised as suicidal, is that forever associated with your name? Who has access to that information?"
Facebook said it had worked with suicide prevention groups when developing the effort.
Elsewhere, Mindstrong Health, a mental health start-up in California, is observing people's smartphone use.
It has developed a research platform to monitor habits, looking at changes in taps and clicks for hints about mood and memory changes associated with depression.
"We are building digital smoke alarms for people with mental illness," said Dr Thomas Insel, a Mindstrong co-founder.
Its research app tracks 1,000 smartphone-related data points - such as how long it takes someone to scroll through a contact list and click on a name.
Dr Insel said a few of the signals, including changes in keyboard accuracy and speed, correlated with similar motor skills changes that researchers could measure in lab tests.
Now, the company is participating in a large government-funded study of trauma patients.
Part of it involves using the Mindstrong platform to study whether patients who go on to develop post-traumatic stress disorder also develop corresponding changes in their smartphone use.
Dr Insel said Mindstrong had tapped law and ethics experts to help examine the implications of its technology and develop ethical frameworks for using it.
"You want to think through all the unintended consequences early on," he added, "so they don't come back to bite you."