If you consume most of your carbs as simple sugars, with negligible fat or quality protein, your blood sugar will peak a bit and then that sleepy feeling occurs as the pancreas secretes insulin to try and correct the misjustice of eating sugary foods. The sleepy feeling is not from lowered blood sugar. It is from the blood supply being diverted to your intestines to fix what you broke. You are merely playing games with the body's natural processes. Juice is sugar.
Sugar is a refined form of a whole food.
If you are not eating whole foods, you are bypassing digestive systems and that is what throws the body into a hormonal disarray. If you consume more calories than you use, then you will gain weight. Weight loss occurs when you use more calories than you consume. What you eat must digest. It cannot if you eat in the Zone.