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There are plenty of climbs/descents they'll have to do at night during the whole ride, Ventoux isn't that bad* in the grand scheme of things as it doesn't have as many sheer drops off the side, or hairpins, as some of the climbs they will encounter on the ride do.
* Saying that, the only time I've climbed Ventoux (from Bedoin) I saw a dead cyclist in the road who had crashed descending. Our descent (to Malaucene) was a lot more cautious than I would normally descend (and I descend like Thora Hird in a Stannah Stair Lift).
Better than without lights...