A very common problem in Zazen, particularly for beginners, is apparent fatigue. You often see people sitting, their heads, which feel heavy, gradually drooping forward, then they notice it and come out of it with a startle. Then they often stick their chest out and lift their head in a rather unrelaxed, mock heroic posture. And then you can see them gradually collapsing again like a slow motion souffle, then taking up the heroic position again; up and down, like an accordion, often for the length of their sitting.
What’s helpful for fatigue in Zazen is to think of it in a different way. It’s not really about fatigue but about having a spine that’s insufficiently activated. If it were really about fatigue, the practitioner would be exhausted before and after Zazen, and they hardly ever are.
Rather than trying to make yourself do something with your mind, whose imperative is to urgently resume the ‘correct’ posture without anyone noticing, it’s much better to make sure that your pelvis is in the right position; that your weight is going down through your sit bones. Then, just very, very finely and slowly, rock backwards and forwards on your sit bones.
Going from the back of your sit bones to the middle through the front very slowly, minutely.
As you’re doing that, on the in-breath you’re pushing down with the pelvic floor at the perineum, and on the out- breath you’re allowing the spine to lengthen. You’re not pushing up the top of the head, you’re not stretching the back of the neck, you’re just allowing the spine to lengthen and be itself, like a young, unencumbered tree.