Like is already stated, cold air is denser air. Colder denser air, loves to make horsepower. Too cold (Dead of night, 12pm-3am or so) and the engine doesn't produce peak power, but slightly warmer in the sweet spot (4am-8am or so) produces peak power.
All fastest laps at lemans have been set around the break of dawn, its a fact anyone whose watched the race more then once will attest to and research would probably show to be true.
Also, the Peugeots in the last race were all making peak power when their banks blew chunks of cheap french engine parts all over the track. I believe all but one went up under cloak of darkness, maybe two.
Drivers hate driving at night. Theres several more distractions, ground fog, mist and vapours from warm tires, trailing smoke and lights from stands and campfires, slowcars, The glowwing vests of safetyworkers and flagmen and the general darkness. Headlights may have improved, but your still only seeing the relative vicinity and not the big picture. Drivers are happy to drive fast at night if pushed, because its the best time to make up time. Not because of a lack of distractions.