Making an interval [i.e. eating; drinking; talking] after saying Kerias Shema Sheal Hamita/Hamapil:
One may not initially make an interval of talking, or eating/drinking, after the recital of Kerias Shema, even if he did not yet say Hamapil, and certainly if Hamapil was already said. Nonetheless, if an interval was made after Hamapil, the majority of Poskim rule that it is not considered a blessing in vain.
A silent Hefsek: Some Poskim say that due to the need to avoid making an interval, one may not recite Kerias Shema [i.e. the blessing of Hamapil] until he feels himself drifting asleep. Other Poskim, however, rule that there is no need to wait until this point, and one is rather to read it immediately [upon resting in bed], lest one fall asleep prior to saying it. Practically, the final ruling follows the latter opinion.
May one eat, drink or talk after Kerias Shema/Hamapil, in a time of need?
If one recited Kerias Shema but did not yet recite Hamapil, then it is permitted to speak or drink in a case of need. However, he is to repeat the paragraph of Shema afterwards. If however, he already recited Hamapil, then it is subject to the above-mentioned dispute regarding if doing so is considered a blessing in vain. Accordingly, some Poskim rule he may no longer drink or speak due to it being an interval, even in a case of great need. Other Poskim, however, rule that one may eat, drink or talk when necessary, even after saying the blessing of Hamapil. Practically, the final ruling follows the latter opinion, however one should only do so in a time of great need.
Based on the above, the following may be recited after Hamapil, if one forgot say it beforehand:
· Sefiras Haomer.
· Asher Yatzar after using the bathroom.
· Kiddush Levana.
· Heard thunder or lightning.
If one made a long interval after saying Hamapil, is the blessing to be repeated?
If one made a long interval after saying Hamapil, he is to repeat the paragraph without saying the concluding sentence which includes Hashem’s name. One is to also repeat the saying of Shema.
If one fell asleep after saying Hamapil, and then woke up, may he eat or drink even initially?
Some write that if one went to sleep with intent to sleep a set sleep, then even if he woke up after a few minutes, he may even initially eat or drink. Certainly, if one slept a set sleep [i.e. at least 30 minutes], he may even initially eat and drink upon awakening. Upon returning to sleep, he is to repeat the paragraph of Shema and Hamapil, without saying the concluding sentence, as stated above.
May one read a book, or learn Torah after reciting Kerias Shema/Hamapil?
One may read a book, or learn Torah in his thought. One however is not to verbalize the words. If the words are verses of Torah that contain powers of protection, similar to the verses in Kerias Shema Sheal Hamita, then it may be recited.
Should a mother abstain from saying Hamapil if they have a nursing child who may prevent them from falling asleep?
Hamapil is to be said even in such a case, as stated above, that the main ruling follows that one is not to abstain from reciting Hamapil due to worry that he may need to make an interval.
 Rama 239:1; Beis Yosef 239; Kol Bo 29 “All Israel is accustomed…”; Rokeaich 327; Rabbeinu Yerucham Nesiv 3:2; Orchos Chaim Kerias Shema in name of Rabbeinu Asher; Sefer Haminhagos of Rabbeinu Asher p. 17; Rabbeinu Manoach Tefila 7:2; Leket Yosher p. 45
Letter of law: The Poskim imply that from the letter of the law, it is permitted to speak and make an interval after Hamapil, as it is no different than speaking after the blessing of Hanosen Lasechvi Bina, prior to hearing the roosters crow, and it is only due to custom that an interval is not made after Hamapil. [See Leket Yosher p. 45; Implication of Rishonim ibid who write it as a mere custom, or custom of some; All Poskim in future footnotes; Tehilas Chaim 2:41 in name of Kuntrus Alei Terufa that this prohibition is not mentioned in the Rishonim, and the Rama brought it as a mere custom, and it is better to speak many times after saying it than to not say it at all; Orchos Chaim Spinka 239:3 in name of Meorei Or that the custom is to drink and speak when necessary]
 Seder Hayom, brought in M”A 239:3; Birchas Habayis 31:2;
 Kneses Hagedola 239, brought in M”A ibid; Elya Raba 239:3; Machatzis Hashekel 239:3 and P”M 239 A”A 3; Chayeh Adam 35:4; Ashel Avraham Butchach Tinyana 239; Maharitz Chayos Brachos 11b; Bireich Es Avraham p. 102 that so is the custom; Yifei Laleiv 239:2; Tehilas Chaim 2:41 in name of Kuntrus Alei Terufa [brought in previous footnotes]; Orchos Chaim Spinka 239:3 in name of Meorei Or [permits even initially to talk]; Birchas Habayis 31:2; Kaf Hachaim 239:7
 Sitting idly on the bed until one falls asleep is not considered an interval. [M”A ibid]
 As so rule majority of Poskim, brought in previous footnote, and so would apply according to all Poskim in coming footnotes, who permit eating or talking after Hamapil
 Implication of Rama 239:1; Siddur Yaavetz; M”B 239:4
 M”B 239:4; Ben Ish Chaiy Pekudei 12 “My custom, and the custom of my forefathers, as well as many of Israel, is not to say the blessing of Hamapil with Hashem’s name, lest we speak in between, and some Poskim rule that if one speaks it is a blessing in vain.”; Possible way of learning Admur 6:8 and M”A 6:8 who rule regarding one who slept, awoke and then went back to sleep that Hamapil is to be recited without a blessing: This is based on the Seder Hayom who rules that Hamapil is similar to Birchas Hanehnin, and we thus see that Admur ibid does suspect for an interval
 Makor Chaim ibid; Siddur Yaavetz; Tehilas Chaim 2:41 in name of Kuntrus Alei Terufa [brought in previous footnotes]; Orchos Chaim Spinka 239:3 in name of Meorei Or [permits even initially to talk]; Birchas Habayis 31:2; Piskeiy Teshuvos 239:3
 As so rule majority of Poskim, that it is not considered an interval, and so is the testified custom, to which the rule of Safek Brachos Lihakel does not apply. [See Kaf Hachaim ibid] Nonetheless, from the fact Admur ibid records the opinion of the Seder Hayom regarding repeating Hamapil, it seems that one should only rely on this in a case of great need.
 Makor Chaim of Chavos Yair 239; Piskeiy Teshuvos 239:3
 So rule regarding if one slept, awoke and then went back to sleep that Hamapil is to be recited without a blessing: Admur 6:8 based on M”A ibid, based on Seder Hayom; Beir Heiytiv 6/7; Machatzis Hashekel ibid; Derech Chaim 227; Ketzos Hashulchan 5/5 footnote 10
Other opinions: Some Poskim rule the blessing is to be repeated. [Siddur Yaavetz] From other Poskim it is evident that the blessing does not need to be repeated at all. [So is evident from all Poskim ibid who rule an interval does not invalidate the blessing; Piskeiy Teshuvos 239:3 footnote 4 and 19]
 Implication of Rama 239:1; Siddur Yaavetz; M”B 239:4
 Tefila Kehilchasa 20:18; Piskeiy Teshuvos 239:3
 See Admur Admur 6:8 and M”A 6:8
 Divrei Shalom 6:35; Piskeiy Teshuvos 239 footnote 21
 See Rama ibid; Admur 61:9