Why Though?

I've maybe talked about this before but why are night time bed sleeps so different from daytime couch nap sleeps? 

Like I try not to nap much these days as it can mess up my night time sleeps but when I do nap I just like, put stuff down and roll over and think "I'm going to take a nap" and I'm out.  Honk zzzzz honk zzzz a SLEEP.

But a lot of times in bed at night?  I'm tired, I am SO ready for sleep and I put down my book and think "I'm going to sleep now" and I turn out the nights and I roll over and nothing.  The sleep just doesn't.

Is there some way I can trick myself into thinking it's just a nap and then sleep all damn night?  Like WHAT is the deal with this?  Why can't I fall asleep at night in bed the same way I can do for a nap during the day.

Does the brain really just "know" stuff?  Is it a cyclical thing?  Hormones?  Circadian?  Light?  WHYYYY YOU BE LIKE THIS?  (Is this just me?)