What time do you knit? IS it near bedtime? if so, then knitting is the perfect pre-sleep timer for you. Maybe start a bit earlier and end a bit earlier than you want to.
Knitting for me is meditative, and I am a meditator as well. So I knit in the mornings, or between meetings, or when having to wait in the car while DH shops or something....or in the bank lineup.
My suggestion is to change your time or change where you knit.....if yoiu knit in public will you get sleepy?
THe other thing is: according to statistics, mose north Americans are sleep-deprived. We busy ourselves so much, have so many choices and distractions. So if knitting is calming you enough to get some precious zzzzz's, count yourself blessed by it!
(the thyroid check never hurts too)