On a ketogenic diet, your entire body switches its fuel supply to run mostly on fat, burning fat 24-7. When insulin levels become very low, fat burning can increase dramatically. It becomes easier to access your fat stores to burn them off. This is great if you’re trying to lose weight, but there are also other less obvious benefits, such as less hunger and a steady supply of energy. This may help keep you alert and focused.
Those issues can be part of what's known as the “keto flu,” Warren says. Other side effects of the keto diet, all of which are tied to carb withdrawal, can include lightheadedness, nausea, mental fog, cramps, and headaches, in addition to tiredness. Luckily, the keto flu doesn't usually last more than a week—which is coincidentally about when people start to see the number on the scale go down, says Warren.
If you eat fewer calories than you burn, you'll lose weight. But when you're hungry all the time, eating fewer calories can be a challenge. "Studies show people who eat 4-5 meals or snacks per day are better able to control their appetite and weight," says obesity researcher Rebecca Reeves, DrPH, RD. She recommends dividing your daily calories into smaller meals or snacks and enjoying most of them earlier in the day -- dinner should be the last time you eat.