Since you're just starting out on elavil, I would wait and take it at night before bed. You are on a low dose, however, all people respond differently to drugs. Elavil, I know at higher doses can make a person pretty drowsy. It is hard to say at 10 or 20 mg, but who knows. To be on the safe side you should wait until bedtime.
I've never really heard of anyone losing weight on Elavil; it is usually weight gain that is the problem.
Hope it works out for you.