Not impossible. If true, I would say it's more likely a contributing factor to mental decline due to dementia. The (Alzheimers Society)[https://www.alzheimers.org.uk/about-dementia/symptoms-and-diagnosis/sleep-problems-treatments-dementia] notes that:

Dementia can disrupt the body’s normal cycle of day and night and their body clock. This means that many people with dementia feel tired and sleepy during the day. They may take many naps that then make it harder for them to sleep properly at night.

Falling asleep in court combined with his well known predilection for posting to the internet at all hours suggests to me that this is a very real possibility.

Additionally here is a post about Trump's mental condition from Harry Segal, senior lecturer from the Psychology Department and Psychiatry Department at Cornell University at Weill Cornell Medicine. While no one can say for sure since no one who can be trusted to tell the truth has had the opportunity to examine Trump, Harry Segal is an expert in the field and his opinion should carry some weight.

“In the 2024 election, both party candidates have been accused by the other of having psychological deficits. This weaponizing of clinical concepts is no doubt confusing to voters and journalists. A careful consideration of these claims can demystify their use as ‘opposition research.’

“Objectively, Donald Trump has a documented history of lying that is so marked as to be considered 'pathological.' Politicians often say untrue things, but the frequency of Trump’s lying is so extreme as to meet criteria for sociopathic behavior, since he tells falsehoods across all life domains, from his personal relationships to business dealings and finally to politics. Recently, several clinicians have noted the ways he has begun to mistake words, lose his train of thought, confuse Biden with Obama, particularly during long rallies held in the evening. There are examples of phonemic paraphasia – swapping parts of words for others that sound similar; these are signs of early dementia, even though they are intermittent.

“The complaints about Joe Biden have been more vague – citing his advanced age, physical fragility and occasional mixing up of names. But there is no evidence of dementia onset, no ‘sun-downing,’ no reports from staff or cabinet members that he is failing to stay on top of legislative or international issues. This in contrast to Trump whose cabinet at the end of his term considered invoking the 25th amendment.

“This is not a partisan take. If the figures here were reversed, I would make the same judgment.”