A power cut at the Bahrain International Circuit stopped track action during day one of the Formula 1 preseason test.
A red flag was issued on Wednesday at around 5 p.m. local time during the first afternoon session after team garages and pitwall monitors turned to darkness.
The floodlights on the track also went out as dusk approached.
The session resumed at 6:10 p.m. local time and has been extended by an hour until 8 p.m. due to the disruption.
A spokesperson for the circuit said: "At approximately 5 p.m. today, a power outage caused by an external substation failure led to the afternoon session on track to be paused. Power has since been restored and the afternoon session has resumed. We appreciate everyone's patience."
The FIA said: "The FIA, Formula 1, and the teams have agreed to extend this afternoon's session by one hour to compensate for the time lost under the red flag. The session will now conclude at 8 p.m. local time."
George Russell told Sky Sports prior to the red flag: "I came to the pits and said 'it's dark out here, I need to change my visor!' Then I realised there are no floodlights around.
"It would have been hectic if this was an hour later and the sun had already set."
Ferrari's Charles Leclerc was top of the times before the interruption with 32 laps completed and a time of 1:30.878.
Mercedes' Andrea Kimi Antonelli topped the morning session.
