I usually begin my baking by feeding my starter with the larger amount of dough and water the night before (around 9-10pm) and start mixing and kneeding the following morning. Yesterday, I fed it during the day, due to time constraings. I spent the whole day doing stuff around the kitchen, and before going to bed I noticed the starter has doubled in side, good news!
However I went to bed and today I woke up to see that the fed starter has gone down. The kitchen was cold overnight, which may have caused that. Is the fed starter good to use? Should I leave it in the oven with the light on? Should I start over?
You should have refrigerated it. Probably still OK to use and I would not hesitate in doing so if it was refrigerated when peaked the night before. Indeed if done so I’d use it even a few days after that. And if the starter percentage of the recipe isn’t so high then even a week or two would still be OK.
Question is that you didn’t refrigerate it but it was colder during the night, it has now fallen so it it fine to use. Probably yes! However if you still wish to bake today and want reassurance then how about taking off 1/3rd of the amount of starter you need for the recipe and then feed it equal parts of water and flour - a 1:1:1 feed - so you’re back to the amount you need. Keep in a warm place and give it 3-4 hours and if in that time it has doubled then its good to go.