statement:
that feeling when you lay in bed after a long day and it just feels so amazing to finally lay down.

second statement:
this feeling is still there quite noticably even after a day that is not quite a "long day"
even just a normal day

question:
what are the specific requirements for creating this feeling?

Part of it comes from vasodilation, which happens after exercise or intense exertion and can give you a really sluggish tired feeling.

The other should be the time-dependent hormones your body makes http://sleepfoundation.org/sleep-topics/melatonin-and-sleep

Hope that helps!

interesting!!
thank you for your information sir

Part of it could be a feeling of accomplishment that generally feels good.