The Be'er Moshe (3:44) says that although the Halacha is that one should not eat, drink, or talk after saying Hamapil before going to sleep, smoking is technically not prohibited and is not considered a Hefsek as far as Hamapil is concerned.
He quotes an earlier gaon that says as follows. Hamapil is considered one of the hundred brachos that a person is required to make everyday. We should not be like those who when it comes time to go to sleep smoke incense and the sleep without saying Shema, rather one should first say Krias Shema Al HaMitah and then go to sleep. Even though one should not be Mafsek between Hamapil and sleeping that is only with sleep and drink and not this.
It should be noted that this Teshuva was written in 1967 when the dangers of smoking were less well known. Additionally smoking in bed is now known to be a very dangerous cause of fires breaking out in the home. We bring this Teshuva only to illustrate the light in which smoking is regarded as a hefsek. Furthermore it should be noted that the issur to be Mafsek after Hamapil is less stringent than other brachos.
Disclaimer: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.
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