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Fatigue and getting sick: cause, or the first symptom?
Does being worn out make a cold more likely, or is the tiredness already the infection starting? What virus-challenge studies and wearable data show, and why looking back rarely settles it.
Measuring heart rate with your phone camera: how it works, and how accurate it really is
Camera-based heart rate uses photoplethysmography, the same optical principle as pulse oximeters. Here's the honest picture of what it can and can't estimate.