Hi there, Linda! We wouldn't suggest leaving the starter out for 18 hours, by overnight we mean roughly 8 to 10 hours. If the starter is left out too long it will start to become dormant and won't provide much for your dough. So say you want to serve dinner around 5 p.m., we'd suggest mixing up your starter around 10 or 11 p.m. the night before so that at about 9 a.m. the next morning you could start making your dough. We hope this helps and happy baking! Morgan@KAF

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