In 1962, he spent two months in the caves of Scarrasson, in the southern Alps. After 61 days on an underground glacier with no time reference, he resurfaced on September 17th believing the date was August 20th.

In 1972, the speleologist supervised other similar experiments before going down again into the Midnight Cave where he remained for 205 days, in collaboration with the Lyndon Johnson Space Center of Houston (NASA). "My brain would make automatic adjustements because it had memorised the previous experiment and yet I was still 2 months out." "We have made a real contribution to chronobiology.But I didn't only study human rhythms out of time, I also analysed sleep, using electro-encephalographic studies."

On November 30th 1999, Michel Siffre, by then aged 60 and a veteran of French scientific speleology, settled in an out of time context in the Grotte de Clamouse. This was how he stumbled into the 21st century. Deep in the bowels of the Grotte de Clamouse, the sixty-year-old pursued his experiments and went through a full range of tests, with the help of his own experience and the latest technologies available. All the indicators concerning his health were monitored live from the surface using a Thomson-designed computer system (deprived of all time markers). Michel Siffre studied the influence of ageing on the way the human body reacts when there is no time bearing. He emerged from the cave on 14th February 2000. The aim of this experiment was to study the influence of ageing on the alterations of circadian rhythms (the alternation between waking and sleep).