Ines ben

Hi! This sounds like it might be hypnagogic imagery, which occurs during the period between sleep and wakefulness, and is linked to the sleep deprivation you described (this article sums it up pretty well). Though the imagery experienced during this period is labelled a hallucination, people are often able to manipulate at least a bit of what they see, explaining why you were able to control what images you were seeing. Some people with aphantasia (including myself) have reported experiences of hypnagogic imagery, suggesting that it is likely linked to different parts of the brain than typical visual imagery! This old discussion post has a lot more information about this phenomenon (as well as some anecdotes of aphantasiacs experiencing it), as does this Reddit post and this article from the World of Lucid Dreaming. 

This is a really interesting question, and research is definitely being done on this! In one of his team's recent papers, Dr. Adam Zeman (the scientist who popularized the term aphantasia) wrote that "aphantasia appears to run within families more often than would be expected by chance," which definitely suggests that there is some sort of genetic link to the lack of visual imagery. You can find the paper and a summary of it here! He also talks a bit about the heritability of aphantasia in his Q&A with the Aphantasia Network if you'd like to check that out as well. As far as I can tell, no one has yet isolated any genes responsible for aphantasia (the research on the genetic link is still in pretty early stages), but people are certainly looking :)