Can one say Krias Shema al hamita early in the evening after Maariv if one is concerned that he will fall asleep before being able to say it.
Krias Shema al Hamita was enacted to say “upon one’s bed” (based on the verse in Tehillim 4:5), and it should therefore only be said close to going to sleep. If one is concerned he may fall asleep, he should say it before entering bed and laying down.
Sources: See Shulchan Aruch, 239:1 and Mishnah Berurah. A further reason why it should be recited at bedtime is the ruling of poskim whereby one must be wary of making a hefsek between Krias Shema al Hamita and sleeping — and certainly for making a hefsek after reciting the hamapil blessing.