As the big Rolex clock at the start/finish line counts down, the teams prepare for the race into darkness.

The sun will set at Sebring International Raceway at 7:38 pm local time, leaving nearly three hours of racing in the darkness.

While fans welcome the flashes of exhaust and the glow of the brake disks, the drivers must adapt to reduced visibility, relying only on headlights, and limited track lighting to illuminate the path around the 3.7-mile circuit.

Working traffic becomes more difficult. The glare of headlights from following cars must be deciphered. Are they class competitors or fleet prototypes?

Is that one car, or two? Maybe it is three. Drivers must try to build a rhythm and recognize critical braking points.

Some drivers thrive in the darkness. Others are not as confident. What of the rookies?

The final quarter has begun.