texan-
welcome to sbd! when you first start the new eating regimen, you will find the lethargy that you were referring to as your body becomes accustomed to the new foods and lack of sugars. your metabolism is switching from using sugars/carbs as the main source of energy. this change takes a couple days. for me this lasted about 2 or 3 days. after that, i had diminished hunger, fewer cravings, and more energy. to help counter act it, be sure to eat all your meals and snacks. if you still feel hungry, add in some more snacks - like some veggies. in essence, be sure to eat enough, don't starve yourself. are you eating plenty of protein and the healthy fats? beans helped me some since they had the carby texture and higher carb content than the veggies. water helps too. again, for me, towards the end of phase 1, i really lost all the cravings for the "bad" foods. stick with it and you will feel great!
check in the SDB Frequently Asked Questions section and you will find tons of answers. Also, the recipe section can add some ideas of good snacks and meals that may aid you in feeling more filled.
congrats again on starting this! you can do it!