'We spent 40 days in a cave without sunlight or watches'
Fifteen people from various backgrounds have recently left a cave in south-west France after 40 days underground.
The experiment was designed to see how the absence of clocks, daylight and external communications would affect the participants' sense of time.
Project director Christian Clot and Marina Lançon, one of the volunteers in the study, told BBC World News about their experience.
