But they are accurate at measuring heart rate, a study of seven devices has found.
The genealogy service says it is changing its terms and conditions after being criticised.
Dr Tedros Adhanom Ghebreyesus chosen as new boss of the World Health Organization
Doctors 'played God' with my body
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The rise of prenatal exercise classes
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Forensic scientists and fire investigators have warned that smoke alarms may not wake children.