There is research support from Aschoff & Wever. They convinced a group of participants to spend four weeks in a World War II bunker deprived of natural light. All but one of the participants displayed a circadian rhythm between 24-25 hrs. Both Siffre's and Aschoff & Wever's study suggest that the natural sleep-wake cycle may be slightly longer than 24 hours. This suggests that exogenous zeitgebers help to synchronise our internal body clock to be associated with our 24-hour day.

A limitation is that endogenous pacemakers and exogenous zeitgebers can't be studied in isolation. Total isolation studies such as Siffre's case study are rare but even then, Siffre used artificial light which may have reset his biological clock. In everyday life, pacemakers and zeitgebers interact so it makes little sense to separate them.