Hi there, Margaret! Once you have a firm idea of how long after your starter is fed that it will start collapsing is when you can start taking this approach. If you live in a warmer environment or it's currently the summer season where you are, this might not be a viable option as the starter could be too far gone by the morning. In this case, you might consider letting the starter rest at room temperature for about 30 minutes to start fermenting and then move it to the fridge to finish ripening slowly overnight. In regards to your question about using the starter once it's started to collapse, the answer is yes, kind of. If you think of the starter as a bell curve, the high point being when the starter is ripest, you can use it when it is just past this point but not fully collapsed. At this point, the dome of the starter will have started to fall inwards slightly but the starter won't have started to recede noticeably down the sides of the crock or jar. We hope this helps and happy baking! 

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