The short answer
For a 100K trail race, practise the headlamp change as a stop, light, secure, verify, move sequence. Stop on stable ground before useful light is lost, switch on the independent backup light, change the primary lamp or battery with both hands, secure every part, confirm the beam, and only then resume.
The current event rules own the required number of lamps, spare batteries, charging method, minimum output, inspection process, and penalties. Some events require two working lamps and multiple spare sets; others use different rules.1 Copy the exact requirement into your race card instead of assuming one universal kit list.
This guide owns the change sequence
The night-running practice guide covers wider darkness, footing, fatigue, and route decisions. The 100K mandatory-kit shakedown covers the full kit. This page focuses on the failure point that matters when the beam fades: restoring reliable light without continuing blind or scattering equipment on the trail.
Verify the exact lighting system
Record:
| Item | Verified answer |
|---|---|
| Required primary lamps | ___ |
| Required backup lamps | ___ |
| Required spare batteries | ___ |
| Rechargeable packs allowed | yes / no / conditions |
| Minimum output or inspection rule | ___ |
| Night section and likely duration | ___ |
| Drop-bag access | ___ |
| Crew assistance permitted | ___ |
| Emergency and withdrawal process | ___ |
Use the official rulebook and athlete guide for the current edition. The Chiang Mai UTMB FAQ, for example, specifies two working lamps and separate spare requirements for certain races.1 That is useful evidence of why exact rules matter, not a rule for every 100K.
Measure useful runtime, not package runtime
Manufacturer runtime can depend on mode, temperature, battery age, regulation, and test conditions. Build a conservative field record:
- charge or install the exact battery;
- mark the lamp and pack so they cannot be mixed up;
- use the intended mode in representative conditions;
- record time until the beam is no longer useful for safe movement;
- stop the test before risking a dark return;
- repeat after several normal use-and-charge cycles.
Do not run an isolated trail until the lamp dies. Test in a controlled place with another reliable light available.
Pack for a two-light change
The backup must work independently of the primary system. Pack:
- primary lamp with the rehearsed battery;
- independent backup lamp already accessible;
- spare battery or charged pack in a dry, protected container;
- a small labelled bag for the depleted battery;
- gloves compatible with the clips and connectors;
- event-required spare items;
- a simple identifier so full and depleted packs cannot be confused.
Keep the backup where you can reach it before removing the primary. A backup buried under waterproof layers is not an immediate lighting system.
Rehearse the six-step sequence
1. Stop early
Use a preselected trigger: visible dimming, a battery warning, a planned time, a known checkpoint, or a beam that no longer reveals the next safe steps. Do not wait until darkness forces the stop.
2. Choose stable ground
Move off the running line only while you still have adequate light. Avoid cliffs, road crossings, stream edges, technical descents, and blind corners. Tell a partner or nearby runner that you are stopping.
3. Turn on the backup
Confirm the backup beam before touching the primary. Aim it so both hands and the immediate ground remain visible.
4. Replace or swap
Remove the depleted component, put it directly into the labelled used-battery bag, install the full component, and close every latch or weather seal. Do not place small parts on the ground.
5. Verify
Check beam, strap, cable, rear battery, and mode. Move your head through the normal range and confirm that nothing loosens.
6. Resume conservatively
Walk the first steps. Let vision and footing settle before returning to the prior pace.
Use a timed practice ladder
| Practice | Setting | Pass condition |
|---|---|---|
| Tabletop | daylight, no fatigue | correct components, no dropped parts |
| Dark room | stationary, backup on | clean change without primary light |
| Easy trail | partner, safe loop | stable stop and secure restart |
| Cold or wet simulation | controlled environment | gloves and seals still work |
| Late-run rehearsal | bounded fatigue | decision happens before beam failure |
Progress only when the prior level is repeatable. Do not create cold, wet, or sleep-deprived risk merely to make a rehearsal feel authentic.
Build a checkpoint battery card
Estimate the night section using the slow end of your trained range, not an aspirational finish. Add:
| Segment | Expected darkness | Primary mode | Planned check | Backup action |
|---|---|---|---|---|
| Start to checkpoint A | ___ | ___ | ___ | ___ |
| A to B | ___ | ___ | ___ | ___ |
| B to dawn | ___ | ___ | ___ | ___ |
Course pace can change with terrain, weather, navigation, aid stops, and fatigue. The card should include margin. It should never justify continuing when the available lighting is unsafe.
Decide where changes are allowed
A checkpoint may offer space, support, or a drop bag, but do not assume it provides batteries or allows crew access. Verify:
- whether the runner must carry all lighting equipment;
- whether crew can touch the kit;
- whether charging is available or permitted;
- whether depleted batteries must remain with the runner;
- whether a mandatory-kit inspection can occur after the change.
Protect batteries from water and accidental activation. Follow manufacturer instructions for charging, transport, compatible cells, and damaged packs.
Account for fatigue and sleep loss
Sleep deprivation can impair endurance performance and decision-making.2 Ultramarathon observations also report altered perception and visual phenomena in some athletes.3 A brighter beam does not diagnose or correct dangerous neurological symptoms.
Use a partner check when possible: name, location, next checkpoint, beam status, and planned action. Stop and seek event medical help for confusion, hallucination, repeated falls, inability to navigate, or behaviour that is not normal for you.
Common failure modes
Waiting for total darkness
Set an earlier trigger. The safest time to change is while enough light remains to choose the stop.
Carrying two lamps that share one failure
An identical connector, cable, charger, or battery may create a common point of failure. Confirm that the backup is independently usable.
Mixing full and used batteries
Use physically separate, labelled containers. Never return a depleted pack to the full-battery pocket.
Practising only with bare hands
Test the exact gloves and waterproof layers. If dexterity fails, change the packing or component before race week.
Assuming cold and water are irrelevant
Temperature and moisture can affect equipment and handling. Use manufacturer limits and a conservative protected setup.
Race-week checklist
- Re-open the current mandatory-kit rules.
- Charge and inspect every permitted lamp and battery.
- Reject swollen, damaged, corroded, or unreliable components.
- Confirm firmware or lock settings without making a late untested change.
- Label full and depleted storage.
- Practise one calm change with race gloves.
- Save emergency and withdrawal instructions.
- Put the backup where it can be reached first.
- Write the stop-before-darkness trigger.
- Confirm the system at mandatory-kit inspection.
Frequently asked questions
Is one spare battery enough for a 100K?
Only the current event rule and your conservative runtime plan can answer that. Some races require more than one lamp and more than one spare set.
Can I use a power bank?
Only when the event rules and manufacturer permit the exact setup. Verify cable security, water protection, capacity, and whether charging counts as the required spare.
Should I use maximum brightness all night?
Use the mode that safely reveals terrain and matches the rehearsed runtime plan. Technical or fast sections may need more light; a low mode is not a success if footing becomes unsafe.
What if both lights fail?
Stop moving, alert another runner or event control, use the official emergency process, and do not continue blindly.
Safety boundary
Night trail running includes falls, navigation errors, weather exposure, traffic, wildlife, and equipment failure. This guide is educational and not medical advice, an event rule, an electrical-safety guarantee, or permission to continue without adequate light.
26weeks.ai keeps night practice, mandatory kit, recovery, schedule, and the 100K finish line connected in one adaptive week. Start training now.
References
- Chiang Mai UTMB. Current mandatory headlamp and battery FAQ. (Official FAQ)
- Lopes TR, et al. Sleep deprivation and endurance performance. (PubMed)
- Martin T, et al. Sleep patterns and strategies in mountain ultramarathon finishers. (PubMed)
- Huang M-K, et al. Visual hallucinations in a 246-km mountain ultramarathon. (PubMed)
- Viljoen CT, et al. Injury and illness among trail runners. (PubMed)
- Mauvieux B, et al. Multidisciplinary protocol during a mountain ultramarathon. (PubMed)
- Garbisu-Hualde A, Santos-Concejero J. Limiting factors during an ultramarathon. (PubMed)
- Tiller NB, et al. Ultramarathon nutrition position stand. (PubMed)