Generally speaking, the more carbohydrate we eat, the higher our blood glucose levels are going to be following eating.
Increased energy
Carbohydrate is often viewed as an energy giving macronutrient but that's only the case if your insulin can keep up. If insulin can't keep up, and this is more likely in people with type 2 diabetes, then our blood glucose levels go too high and we can end up feeling tired and lethargic.
By starting a low carbohydrate diet, you may also be lowering your daily calorie intake. As a result, some people may experience more tiredness than usual shortly after starting a low carb diet, but this effect usually passes after the first 2 weeks of the diet as the body adapts to the new diet.
Improved clarity of thinking
If a low carb diet achieves the desired effect of improving blood sugar levels, it can help to relieve brain fog which can occur when blood sugar levels are too high.
Brain fog is characterised by an inability to think clearly. Many people following a low carb diet report having improved clarity of thinking after starting the diet.