Fatigue and feeling tired are two of the most common side effects when starting a ketogenic diet. For example, for many people switching to keto, their body has been running on an ample supply of glucose for their entire life. Suddenly restricting this glucose supply is a significant change for the body, and in the initial stages, our body seeks this glucose for energy. While these low energy levels are not enjoyable, they are entirely understandable in this context. When our body realizes this glucose supply is not present, it up-regulates fat-burning enzymes and starts producing more ketones. Until the body becomes efficient at using fat for energy, these feelings of tiredness linger. For the majority of people, these side effects last anywhere from 2 or 3 days to a week.