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It's an automatic feature, and I don't think you can adjust it in any way. Many nights my time to sleep is 0, which means I pretty much fell asleep immediately (according to the tracker). If yours is showing 0, it is still logging.
There is a sleep setting you can change from Normal to Sensitive in the app. I'm not sure how or if it would affect the time to sleep data, but you could give it a try.
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