extremely tired next day after first dose?

hi everyone, been struggling with alcohol for years. after a crazy fourth of july bender i finally decided it was time to give nal a try. i’ve had the prescription but never tried the first dose cuz it was never the right time to potentially deal with side effects. i took at last night (12.5 mg) and drank 3 seltzers which is low for me. i started to get tired and feel a little out of it so i went to bed. this morning i feel exhausted (but not hungover yay) and can’t seem to get up for the day. but im wondering if this is a side effect that will lessen over time or something that Nal does consistently. thanks! ps- planning to do 12.5 dose 2-3 more times, then move to 25, and up to 50 eventually.