Smartwatches and rings give a detailed morning summary of the previous night, measuring everything from sleep duration to overall energy levels. While modern technology helps people stay more connected to their health, sleep doctor Heisl Vaher recommends always trusting personal bodily awareness alongside smart devices.

Wrist- and finger-worn devices continuously monitor the user's heart activity, breathing rate and body temperature. Vaher said the growing public interest in health pleases him as a physician, but the trend has an inevitable downside. "We also have people who tend to follow the devices too closely, and that can raise anxiety levels beyond what is healthy," she said.

Smart devices track sleep but often mislead users

Health-tracking devices and apps try to translate complex biological processes into easily understood numbers and colorful charts for user convenience. But the point scores that appear on screens often give users a misleading impression of scientific precision, even though the devices contain a fairly wide and estimated margin of error.

"The problem arises when device data are taken as gospel, without considering that these are not medical devices," Vaher noted.

The limits of the technology are especially clear when machines try to assess sleep stages. Colored bar charts for deep, light and REM sleep rely only on indirect signals collected by sensors. Clinically valid sleep-stage analysis requires direct measurement of brain waves, which home devices cannot do. "I would be cautious about treating sleep stages as highly precise. We still do not have devices that determine sleep stages in a way that would be acceptable for medical analysis," Vaher emphasized.

Smart app algorithms also calculate daytime activity and energy expenditure alongside nightly rest. These programs typically paint an overly optimistic picture and may allow people to consume far more calories than their bodies actually need. "They all overestimate real expenditure by about 20–30 percent, so it's wise to be cautious," Vaher explained.

Sleep doctor warns against overinterpreting nightly scores

To avoid drawing faulty conclusions from screen data, it's worth constantly comparing the displayed numbers with one's subjective sense of well‑being, no matter how high the rest score is. The sleep doctor added: "Watch data should be used together with your internal sense and recognition of how well you slept last night and what your overall sleep experience is like."

Although home devices have clear technological limits, they still show great potential in capturing certain specific biological signals. Vaher said heart rate variability, or HRV, is one of the most reliable data points that smartwatches collect in real time.

This measure directly reflects autonomic nervous system activity and indicates the balance between the sympathetic and parasympathetic systems. "If it's consistently good and high, I would expect the person to be relatively physically active and attentive to their sleep habits," Vaher said.

A responsive HRV provides valuable feedback about what is happening in the body and can warn of an approaching illness long before the first real symptoms appear. Short, one‑day fluctuations in the numbers usually reflect a hard workout or a heavy dinner the night before. When analyzing data, the doctor therefore recommends looking at the long‑term picture rather than reacting emotionally to every small change. "Watches show us trends, and you don't need to be overly disturbed by individual spikes," Vaher clarified.

Irregular bedtimes disrupt the body's sleep‑wake rhythm

When examining longer trends, watch algorithms scrutinize bedtimes, which often vary widely across weekdays for many people. Users frequently try to repay accumulated sleep debt on days off by staying in bed longer. Sometimes an app even rewards longer-than-usual sleep with high scores while ignoring the fact that the person fell asleep only in the early morning. From a sleep‑biology perspective, a consistent bedtime routine plays a far more important role in maintaining physiological balance than absolute nightly sleep duration.

A regular daily rhythm gives the body a clear signal to trigger the biological processes needed for deep, restorative rest at the right time. "Falling asleep and staying asleep depend on the release of certain hormones and substances in the body. This happens according to the sleep‑wake rhythm, and if we disrupt it with widely varying bedtimes, the release of those substances is disturbed," the sleep doctor said.

Hormonal imbalance helps explain why people often still feel groggy after a very long weekend of sleep.

When more serious health problems arise, smartwatches inevitably fall short and can never replace proper medical diagnostics. Persistent physical exhaustion and difficulty coping with daily tasks are much clearer signs of real need for help and sleep disorders than smartphone alerts.

Vaher said: "Daytime well‑being is an excellent indicator. Self‑assessment ability tells an adult fairly quickly when something is wrong."

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