I have lost weight in the last 18 months and I do have more energy. I have increased my exercise and I believe that this on its own will give you more energy. I am not one for naps, which I presume is the kind of sleepiness you mean. I do sleep better if I’ve had a busy energetic day.
I think that if you don’t eat enough and your body starts reacting to this, you will feel more tired and lethargic. This is because your body is trying to conserve energy. This will happen on a low calorie diet. The trick is to reduce calories enough that you lose weight but not so much that your body goes into energy conservation mode.
Crash diets are so aptly named. Not only do you crash the number of calories you intake, you crash your energy levels and then your diet crashes meaning you start gorging and put all that weight and more back on again.