Unfortunately there's no way for people with complete aphantasia to "train their brain" to think in images. When they've done MRIs on people with aphantasia, the visual cortex doesn't light up--it just isn't active without stimuli. It sounds like you're maybe a 4 on the scale and have some visualization ability, which is maybe why you are able to see some things when drowsy. I hope you find clarity in your journey if you want to try to increase that ability, but for those of us with nothing at all, asking us to "try to conjure images when sleepy" is like asking a fish to climb a tree.