Why does my automatic watch stop overnight?

An automatic that stops overnight has run out of stored energy, and it ran out because it never got near full wind in the first place. A full mainspring in a typical modern automatic holds roughly 38 to 45 hours, and 41 hours is the published figure for the widely used Seiko NH35. A watch that dies in eight hours on the nightstand was nowhere near that.

Two explanations account for nearly every case: your day is not moving the rotor enough, or the winding system is not converting what movement there is. Telling them apart takes about a week and costs nothing.

The arithmetic of a power reserve

Think of the mainspring as a tank with a constant drain. The movement draws down its reserve at a fixed rate whether you are wearing it or not. The rotor tops the tank back up, but only in proportion to how much your arm actually swings.

If those two roughly balance, the watch hovers at whatever level it started at. It never climbs to full, so it never has a night's worth of margin. Owners in this position often describe the watch as fine all day and dead by morning, which is exactly what a permanently half-full barrel looks like. It also explains a second complaint that usually arrives with it: erratic timekeeping. Mainspring torque falls as the spring unwinds, balance amplitude falls with it, and rate becomes least stable in exactly that low-power region.

Desk diving: the usual culprit

Manufacturers quote power reserve from a fully wound spring, and rotor efficiency is measured on a machine that swings the watch through a defined arc for hours. Neither matches a day spent typing with your forearms resting on a desk, driving with your hands on a wheel, or sitting in meetings. Small, damped movements barely turn a rotor.

Other habits do the same thing:

• Short wear time. A watch on the wrist from 8 a.m. to 6 p.m. has ten hours to bank the fourteen it will spend on the nightstand.
• A loose bracelet. A watch that slides on the wrist absorbs motion instead of transmitting it to the rotor. Our guide to sizing and fit covers how snug is snug enough.
• Rotating a collection. Any watch worn two days out of seven will be stopped more often than not, which is normal and harmless.
• A micro-rotor caliber. A smaller weight closer to its pivot generates less winding for the same motion, by design.

When it is the watch, not your day

A watch that runs down after an active day, or that fails the one-week test, has something wrong in the winding chain or the movement itself. These are the usual findings on the bench.

A timing machine settles most of it in minutes. A healthy modern movement shows amplitude somewhere around 270 to 310 degrees dial up shortly after full wind, and should still be comfortably above 200 degrees a day later. A watch reading well below 200 at full wind has a real problem, and stopping overnight is only its most visible symptom. Our timegrapher reading explainer walks through what the numbers mean, and timing your own watch covers doing it yourself.

Small things worth ruling out first

Before you book a service, check the cheap explanations. A watch that stopped at the same time on consecutive nights may be catching on a date change with too little power to complete it. A movement recently exposed to a strong magnet can run wildly fast rather than stop, so magnetism is rarely the answer here, but it is worth excluding if timekeeping went strange at the same moment; see can magnets damage my watch. And if the watch is new, or newly serviced, the fault belongs to whoever worked on it last.

Finally, be honest about the age of the movement. A watch that has never been serviced in ten or fifteen years is running on degraded oils, and a shrinking power reserve is one of the classic ways that announces itself. Our answer on service intervals covers when that clock starts.