The aim

1. The question was what happens to the 24-hour sleep-wake cycle when someone lives underground with nothing to tell them the time, leaving them on a free-running rhythm.

Going underground

1. It was a case study of one man aged 33, Siffre himself, who lived in a cave in Texas for six months, entering on 14 February and leaving on 5 September, though the final four weeks were not spent in isolation.
2. He telephoned a team on the surface when he woke and again when he felt sleepy, and they switched the lamps to match.
3. He tested his own blood pressure, mental sharpness, memory and dexterity on a regular routine.

The results

1. His sleep-wake cycle ranged from 18 hours at the shortest to 51 and three-quarter hours at the longest.
2. Every one of those cycles felt to him like a single day, however long it actually ran.
3. A 48-hour cycle became common across two extended periods of the stay.
4. His memory, mood, dexterity and clarity of thought all deteriorated over the months.
5. At the end of the sleep part of the study he believed it was mid-July, when the date was in fact 10 August.

What the study concluded

1. Without zeitgebers the sleep-wake cycle does not merely stretch a little, it drifts a long way from 24 hours.
2. The rhythm still ran, which shows it comes from inside the body, but nothing was holding it to the length of a day.
3. The outside cues are therefore what keep an internal rhythm in step with the world, rather than what creates it.

Strengths of the study

1. A strength is that it produced a unique set of long-duration data on a free-running rhythm, which no short study could match.
2. A second strength is that he telephoned the surface at every waking and every onset of sleepiness, so each cycle was timed from outside rather than left to his own sense of how long a day had lasted.

Weaknesses of the study

1. A weakness is that there was a single participant, so his cycle lengths cannot be assumed to be typical of anybody else.
2. A second weakness is that he was exposed to real risk to his physical and mental wellbeing, which raises a serious question about protection of participants.

Turning this into marks

1. Name Siffre and give the six months alone in a cave in Texas.
2. Quote the cycle range of 18 to 51 and three-quarter hours, and the 48-hour cycle that became common.
3. The mark most often lost is giving the figures without saying that the body clock kept running but lost its 24-hour length.