Often the problem can be related to what you ate at your last meal. If you are feeling low on energy in the afternoon , it could be because of what you ate at lunch that is effecting your blood sugar levels. You ate too much or ate too many carbohydrates.  I cut down to eating a half sandwich and 1 piece of fruit or vegetable and I feel much better in the afternoons now.   You may also want to consider getting tested for Hypoglycemia.  I was diagnosed with that and since I changed my diet I am relatively symptom free.  No DONUTS!