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One who fell asleep during the reading:
The definition of dozing versus falling asleep: In this law there is a difference between one who fell asleep during the reading and one who simply dozed off in a “half sleep” mode, as commonly occurs to listeners during a speech and the like. The definition of a “doze” in this respect is one who is only “half asleep” and is hence able to answer a question if he is asked, and although he is unable to give any intellectual argument to his answer, if he is told the argument he will immediately remember it.
The ruling: If one began dozing off during the reading, then if he is the reader [and the words were clearly verbalized] he [and the listeners] fulfills their obligation. However if a listener began dozing off during the reading, then he does not fulfill his obligation. [Nevertheless if he is following along, and verbalizing the words silently, in a Kosher Megillah, then he has the same status as the reader and he fulfills his obligation.] If however the reader actually fell asleep then the reading is invalid and he must repeat the reading.
Lechatchilah: In all cases that the reader dozed off and continued reading he is initially to be awoken and made to repeat the words that were said while he was dozing. The above law is hence only applicable in a case of Bedieved that the reader already completed the reading, in which case we rule he fulfills his obligation.
One must be very careful not to fall asleep or doze off during the reading. If one fell asleep or dozed off then if this occurred to a listener it is considered that he missed those words read during his doze and he thus does not fulfill his obligation. Practically, if this occurs in middle of the reading, he is to read the missed words until he catches up to the reader, as explained in Halacha 15.
 Megillah 18b; M”B 690/39; Kaf Hachaim 690/68
The end of the doze: We rule there is no difference in law between the first moments of his doze and the end of his doze. [Taz 690/8; Biur Halacha 690 “Karah” unlike Bach who invalidates the end of the doze being that he is too deep into his sleep] Practically this matter is dependent on each person, that if he feels he was able to say the words properly, even towards the end of his doze, he fulfills his obligation.
 M”B 690/40
 M”B 690/40 based on Meiri; Kaf Hachaim 690/71
The reason: As the reader is able to intend to fulfill their obligation even while he is dozing off. Alternatively his intent for the listeners to fulfill their obligation by the start of the reading suffices even during the time that he is reading in a way that he is dozing off. [M”B ibid]
 Michaber ibid
The reason: The reason for this is because certainly the listener missed some of the words of the reader. However a reader who is dozing off certainly fulfills his obligation if he continued saying the words properly, as this proves he was awake enough to consciously say the words. [P”M 690 M”Z 8; M”B 690/41] Alternatively the reason is because the listener is unable to intend to fulfill his obligation while he is dozing off, and hence even if he still hears the words he does not fulfill his obligation. However the reader fulfills his obligation even if he dozed off in middle as the fact that he continued reading shows his ability to have intention. [P”M ibid based on Beis Yosef and Yerushalmi; Kaf Hachaim 690/72; however see Shaar Hatziyon 690/39 that one must only have intention at the beginning of the reading, as brought in M”B 690/40]
 Implication of Michaber ibid
 Birkeiy Yosef in Shiyurei Bracha 690; Beis Oved 690/10; Kaf Hachaim 690/70