Mercedes broke the mechanics' curfew overnight at the U.S. Grand Prix in order to make a precautionary change of the fuel system on Lewis Hamilton's car.
In order to prevent team members working unnecessarily long hours, the FIA imposes an eight-hour curfew between Friday and Saturday for paddock personnel working on the cars. In Austin the curfew ran between 23:00-07:00 but Mercedes mechanics worked within that time to fit a new fuel system.
Teams get two exceptions from the curfew per season, so no penalty will be imposed on Mercedes which had not broken curfew at any point this season ahead of this weekend.