Is there a latest time one can recite Shema before bed?
For example, if someone naps during the day and doesn’t wake up until the early hours can they say Shema and Hamapil then? Or likewise, if someone works shifts, if they work a night and go to sleep in the day, should they still recite some of the prayers about protection during sleep?
Birkas Hamapil is only when one is going to sleep at night, therefore if one goes to sleep by day, even if he continue sleeping at night he does not say hamapil. The reason for this is, since hamapil is meant to be said when it is the time that people go to sleep, similar to the bracha of Hashkiveinu, which are not said during the daytime. (Night time for this is alos hashachar, daybreak, which is about 72 minutes before sunrise.) If he lays down right before daybreak, and he isn’t sure if he will actually fall asleep before daybreak, it is questionable and if wants he may say it without saying H-shem’s name.
M:B 239-8, Biur Halacha ibid D:H Somuch, Shut Keren L’dovid 1-60. Shut BIinyan Dovid 29.