Life Cycle of Brewers Yeast:
1. Respiration - Also known as lag time. In this stage, the yeast is reproducing to reach critical mass, which then leads to the second stage, fermentation. You want to minimize the time the yeast spends in this stage because you want the yeast to start alcohol fermentation as soon as possible to prevent invasion and spoilage from bacteria or wild yeasts. This is why proper sanitation and pitching healthy brewers yeast or a yeast starter is essential.
2. Fermentation - In this stage, the yeast is actively consuming the sugars in the wort, turning them into CO2 and alcohol.
3. Sedimentation - The yeast enter this stage once the sugar level starts to drop. The yeast drop out of solution and become dormant, flocculating (dropping out) to the bottom of your fermenter or bottle forming sediment. Again, the yeast go dormant, they do not die. If additional sugars are added the yeast will go full-throttle again.