שמואל ב, פרק י״ג, פסוק ד׳

A sharp observer can often read the physical toll of a hidden emotional struggle. Jonadab notices the deteriorating condition of the king's son and approaches him with a question that reveals a deep understanding of human psychology. The primary approach among commentators is that Amnon appears physically thin, weak, and miserable. Jonadab pays close attention to the timing of this condition, noting that Amnon looks his absolute worst every single morning. Appearing so unwell before the day has even begun points to a profound inner distress that shatters his rest [ביאור שטיינזלץ]. Thoughts of Tamar consume his mind throughout the night and rob him of his sleep, leaving him pale and exhausted by dawn [רד״ק, מצודת דוד].

Recognizing these symptoms, Jonadab correctly diagnoses a severe case of lovesickness that is causing Amnon to waste away. Cornered by this accurate deduction, Amnon is forced to confess his secret [מלבי״ם]. When he finally answers, Amnon does not merely state Tamar's name but deliberately identifies her as Absalom's sister. This detail is a crucial hint about her legal status. He emphasizes this relationship to highlight that she was conceived while her mother was still a gentile, meaning she is only related to Absalom through their mother. Consequently, Tamar is not legally considered David's daughter or Amnon's sister, making it permissible for Amnon to marry her. This very fact—the obsessive realization that she is technically permitted to him, fueled by his fierce passion—is exactly what torments him and drives away his sleep night after night [מצודת דוד].