I say fluctuate between the two ... if you get bored of eating Graham crackers , substitute it with the oatmeal ones to make things interesting. No biggie of a challenge there ... Hope that helped
This is one phrase used through out the Atlas Course , in addition to 'do not do the exercises till exhausted'
Can the experienced D.Ters shed some light on this topic. What is the sensation one is going through till they reach the point where it is mentioned "until tired" ... e.g should the body ...