Apps and gadgets promise to unlock the secrets of rest—but have unintended consequences, reports the Financial Times.

Most trackers rely on movement as a proxy marker of wake and sleep, utilising sensors called accelerometers to measure motion. Recent studies have shown that while many trackers are reasonably accurate at measuring time in bed, they are less good at determining total sleep time, especially if you wake in the middle of the night. As for determining the stages of sleep, from light to deep to REM, as most trackers promise — some are only just better than the flip of a coin.

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