It's well known, and well documented, that prednisone can cause insomnia, but what I can't figure out is how it knows you're horizontal?

Sitting on the couch about 10pm, bit of a yawn fest happening, take some pain killers and some temaze (all of which should cause sleepiness) but lo and behold, within 15 mins of lying down, I'm wide awake and raring to go!

Why prednisone, why would you do that to a girl?!?!