le mans 24hrs isnt quite 24hrs by default either. They generally revolve around 12 and 24 minutes. However.. if your a hardcore driver with an awesome setup and want a close experience.. you can dial the timer up to many ours, up to 24 hours. Also the light cycle seems to be early evening -> night -> dawn.
Also It seems only such a day/night cycle can occur on the ONE Le Mans 24hr track. It would really have been nice to see it on the cities too. Esp since I love the headlights in this game. I dunno if NFS carbon updated them, but NFS Underground 1/2 headlight wasn't all that realistic, and the game only rendered it for your car.
Mobo: EVGA 122-ck-nf68 680i sli, CPU: C2D Quad 6600, Video: EVGA GTX 460 1gb SC, Sound: X-fi gaming, Windows 7, Antec P180b,Thermalright Ultra-120 HSF, Antec TruPower TP3-650, 4 Gigs RAM