What are the reasons for the hallucinations that occur immediately upon waking up at night, such as seeing one's hand as very long, then returning to its normal state upon saying "Bismillah"?

We have no knowledge regarding the cause of these imaginations. You may consult the "Consultations" section of the website or trusted psychiatrists. We advise you to sleep in a state of ablution (wudu'), remember Allah upon sleeping and waking, rely on Allah, entrust your affairs to Him, and supplicate that He avert from you what you dislike. Let your fear and hope be solely connected to Allah alone, and adhere to the remembrance of Allah, for it dispels worries, removes distress, and by it, hearts find tranquility, in accordance with His, the Exalted's, saying: "Unquestionably, by the remembrance of Allah hearts are assured" (Ar-Ra'd: 28).