At UTMB (168 km, 27-44 h), runners rested about 12 minutes on average, and sleep loss plus the effort impaired cognition
From slower reaction times to visual hallucinations, regardless of how long they rested
Participants
n=17 runners completing the Ultra Trail du Mont Blanc (168.0 km); race completion times 27 to 44 h.
Protocol
Wrist-worn actigraph throughout the race to record sleep time.
10-min response-time tests before and after the race; questionnaire on difficulties experienced during the race.
Key Findings
Rest during the race: 12 ± 17 min on average.
The combination of acute sleep loss and 168.0 km of strenuous exercise had marked adverse effects on cognitive performance, ranging from lengthened response time to serious symptoms such as visual hallucinations.
Authors' conclusion: cognitive performance was adversely affected regardless of rest duration and time in race.
Practical Application
In 17 UTMB finishers (168.0 km, 27-44 h), who rested only 12 ± 17 min during the race, sleep loss plus effort lengthened response times and in some produced visual hallucinations — and the impairment occurred regardless of how long they rested or how long they took. Expect degraded judgement and reaction on the night sections of a race through the night; build safety buffers rather than relying on a short nap to prevent it.
Do not treat hallucinations or disorientation as normal grit — they are a stop-and-assess safety signal.
Inference, not tested: the tiny rest times here say nothing about whether a longer planned sleep would help; the study did not compare sleep strategies.
Limitations
Small observational sample (n=17), one event, no control group; response time tested only before and after, not during the race.
Rest was minimal for nearly everyone, so the "regardless of rest duration" conclusion rests on a narrow range of rest.
How this entry was checked and changed
Content correction 2026-09-27
"~46 h" -> race completion times 27 to 44 h (abstract); added n=17, 168.0 km, rest 12 ± 17 min, actigraphy and response-time protocol.
"Supports a planned sleep/nap strategy" -> the abstract says impairment occurred regardless of rest duration; the nap claim removed and the gap labelled as inference.
Cross-reference REF-605 relabelled: it is the travel fatigue / jet lag consensus.