Circadian rhythms are 90 to 120 minutes. There is a rest period of about 20 minutes during each. It is possible to over-ride this need for rest, and it actually becomes more difficult to feel them coming. If you can't actually stop what you are doing, this is a good time to slow down. These intervals are great for standing up, stretching, taking some deep breaths. If you meditate or do self-hypnosis or anything similar, like the Silva Method, these rest intervals are the best time for it. Of course, the first step is learning to feel your body's signals and recognizing these times when it is better to slow down.