A weakness of endogenous pacemakers?

-Can't be studied in isolated -Total isolation studies, like Siffer's cave study are extreemly rare and even in this study Siffre made use of artifical light which could have reset his biological clock every time he turned his lamp on. -In everyday life, endogenous pacemakers and exogenous zietbgebrs interact, and it may make little sense to separate the two for the purpose of research -Suggests tha tmost reseachers attempt to isolate the influnece th internal pacemakers, the lower the validity of the research