Two Nights Around Mont Blanc: Mapping Darkness on the 2026 UTMB

You might know PitStopper as a cycling tool. It isn't only that - it works for any GPX, and most modern sports watches speak exactly the same language as bike computers: courses and course points. Load a route, pin what matters to it, and your watch will buzz you a warning as each point approaches. Which makes it rather useful for the biggest trail race on Earth.

The Ultra-Trail du Mont-Blanc needs little introduction: roughly 170 km around the Mont Blanc massif through France, Italy and Switzerland, with about 10,000 metres of climbing, starting in Chamonix on the last Friday of August at 18:00. It is the race every trail runner has an opinion about.

It's also superbly organised - aid stations every 10-15 km mean nobody needs us to count water taps. So we asked PitStopper the questions the roadbook doesn't answer:

1. Where does darkness actually catch you? Not "bring a headtorch" - which col, which kilometre, at your pace?
2. Where is the nearest roof when Mont Blanc weather does what Mont Blanc weather does, between aid stations, at 2,400 m, at 3 am?
3. And - asked quietly, answered honestly - where are the toilets?

Your pace, not the winner's

PitStopper's time estimate isn't a flat average. For walking it uses Tobler's hiking function - speed falls away on steep gradients, exactly what 10,000 m of Alpine climbing demands - plus a rest buffer it added automatically when it saw the climbing total ("+10% rest buffer for 10,064 m climbing", it said, unprompted). Feed it a flat-ground base speed and it walks the whole GPX, gradient by gradient:

(The elites finish in around 20 hours and get just one night. The rest of us get two - and I say "us" generously, because I'm not actually a runner myself. The cyclist in me winces at doing this sort of distance without wheels. This post follows the 43-hour mid-pack runner.)

We set the clock: Friday 28 August 2026, 18:00, Chamonix. Then one click - Add Sunrise/Sunset POIs - and PitStopper pinned dawn, sunrise, sunset and dusk onto the route itself, at the point where each will happen at that pace, across all three days.

Night one: the Bonhomme in the dark

Read that first row again: the sun sets 10 km in. You leave Chamonix at 18:00 to church bells and cowbells, and by Saint-Gervais the headtorch is on. Night one is 34 km long and contains the entire climb over the Col du Bonhomme (2,329 m) - the race's first big test, done blind. The one mercy of a late-August UTMB: a 94% moon is up there with you.

Dawn arrives at km 46.1 - which happens to be almost exactly the Refuge du Col de la Croix du Bonhomme at 2,449 m (km 46.5, arrival 06:23). You crest the first great col of the race as the sky lightens over the Beaufortain. Plan the emotional low for 4 am on the climb; plan the photo for the top.

Night two: Grand Col Ferret by headtorch

Saturday is the Italian day - Col de la Seigne at breakfast, the Vallée des Glaciers behind you, Courmayeur in the afternoon heat, then the balcony path past Bertone and Bonatti as the light goes golden. And then:

Sunset lands at km 101.3 - at the foot of the Grand Col Ferret climb, just past Rifugio Elena. The highest point of the entire race (about 2,530 m, the Italian-Swiss border) goes by in full darkness under a waning 87% moon. Night two is the long one: 43 kilometres - the whole Swiss Val Ferret, La Fouly, Champex-Lac, the grinding Bovine climb - before dawn finds you at Trient (km 145), two climbs from home. Finish line: Sunday, 13:02.

Add it up and the mid-pack runner spends 77 of 172 kilometres in the dark - 45% of the race. The roadbook says "night sections"; the map says which ones, for you.

When the weather turns: 17 refuges and 24 shelters

Between aid stations, the high cols are committing - and Mont Blanc storms don't check the race schedule. So we mapped every roof within 200 m of the trail: the mountain refuges (via a custom tourism=alpine_hut tag - 30 seconds to add in PitStopper, no built-in category needed) and OSM's emergency shelters.

Every refuge, with your mid-pack arrival time:

The pattern is reassuring in Italy - a refuge every hour or two along the balcony - and thinner exactly where the nights are: one roof (Plan de l'Au, 02:32) in the 34 km between La Peule's side of the Ferret and Trient.

The emergency shelters fill some of those gaps, and one deserves a special mention: the Abri du Col du Bonhomme, a stone bothy at 2,330 m, sits at km 44.5 - which the model has you passing at 05:45, the coldest hour of night one, 2 km before the dawn. If the weather is filthy on the Bonhomme, there is a door to put between you and it. There's another high one at the Lac des Vesses (2,333 m) on the Mont Favre balcony, and a cluster around Trient right where night two ends.

And yes, the toilets

53 public toilets sit within 200 m of the course. They cluster where you'd guess - ten around Les Contamines' park and aid station zone (km 32-36), a wall of them through Courmayeur - and vanish where you'd fear: not one for the 20 km between Les Chapieux and Lac Combal (the Col de la Seigne crossing), and a 13 km gap through the Bovine forest in the small hours of night two. Between those, there are 2,500-metre cols and not much else. Plan accordingly; leave no trace.

The aid stations came free

One more thing we didn't expect: the official UTMB GPX carries its 35 checkpoints as a route inside the file, and PitStopper imported them straight onto the line as POIs - every control, distance-pinned, ready to export.

This is where the watch earns its place. Export the route as a FIT course (or GPX) and every POI you kept becomes a course point - the same mechanism bike computers use, and one that Garmin, Suunto, Coros and friends support on the wrist. Deep into night two, you don't have to remember whether the next roof is Plan de l'Au or Trient: the watch buzzes "shelter, 300 m" and that's the whole navigation. Refuges, shelters, checkpoints, even the sunset markers - they all ride along.

Caveats, honestly

The clock times are a model - Tobler's function at a 6 km/h flat base with a rest buffer, not your race plan. Aid-station stops, sleep, blisters and heroics are not in it; treat every time as "roughly here, roughly then" and it will still tell you more than "bring a headtorch". Refuges keep seasonal hours and some are unstaffed; an OSM shelter tag spans everything from a solid bothy to three walls and an opinion. Daylight times are for late August 2026 at the route's latitude; the moon data is real (94% then 87% - a lucky year). Data © OpenStreetMap contributors, snapshot July 2026.

Do this for your own race

Everything above took one GPX and about five minutes in PitStopper: load the file, set your start time and pace (walking mode for mountain routes - it understands gradient), click Add Sunrise/Sunset POIs, and search whatever categories your race worries about - we used shelters, toilets and a custom refuge tag. Then send it to whatever you race with: bike computer or sports watch, if it takes a course, PitStopper can feed it - course points, alerts and all. Runners, hikers, bikepackers - the mountains will still surprise you. The map doesn't have to.

Route data from the official UTMB stage GPX. POI and refuge data © OpenStreetMap contributors. Sunrise, sunset and moon phases are computed entirely in-app (SunCalc and lunarphase-js) - no API, no network, works offline.