This may sound strange, but sometimes if you do something to fall asleep - like reading in bed, or reading to wind down prior to falling asleep, your brain will associate that activity with bedtime & will produce the chemicals necessary to sleeping whenever you perform that activity.

(I've fallen asleep during a particular cartoon that's on late at night so many times, & now even when my husband tapes it for me & I try to watch it earlier in the evening - bam, I'm out like a light.)