Consumer wearables estimate sleep from heart rate, its variability, movement and skin temperature, not from brain activity. Against polysomnography, the clinical gold standard that records brain waves, eye movements and muscle tone, they are fairly accurate on a basic question: a 2025 meta-analysis of 24 studies found they overestimate total sleep by about 17 minutes a night. Their weak point is quiet wakefulness, when a person lies still and awake but gets scored as asleep. Sleep-stage tracking, such as how much deep or REM sleep you had, is much less reliable, and studies of six wrist devices found only modest agreement. Trackers do capture total sleep duration well, which matters: a Fitbit study of over 600 students found that those sleeping six hours or less a night early in term had lower grades later.

Hampi is a UNESCO World Heritage Site in Karnataka, India, with more than 1,600 surviving remains spread over 4,100 hectares. It was the capital of the Vijayanagara Empire from 1336 to 1565. Travellers from Persia and Portugal described it as a prosperous, wealthy and grand city with temples, markets and farms along the Tungabhadra River. By 1500 it was estimated to be the world's second-largest city after Beijing, and probably India's richest at the time. In 1565 a coalition of Muslim sultanates defeated the Vijayanagara Empire, its capital was conquered and destroyed, and the city was left in ruins. Hampi remains a religious centre, with the active Virupaksha Temple at its heart.