An analogue clock, a clock with hands, shows the numbers 1 through to 12. It is a 12-hour clock. The first 12 hours of the day, from midnight to midday, are called am. The next 12 hours ...

O’Malley’s team also found that a 12-hour clock, distinct from the 24-hour circadian clock, drives this morning-evening rhythm in gene expression. The clock’s origin, the scientists suggest, may be ...