Unable to sleep after relaxation exercise(?)

Slightly offtopic but I wonder if anyone has any thoughts on this.. 99% of the time I sleep absolutely fine unless I'm particular stressed about something. I'll usually be asleep within about 15 minutes of lying down.

Last night I was doing my relaxation exercise for the first time in a long time. Essentially it consists of just lying flat without moving and doing 10 to 1 rundowns focusing on sinking and relaxing with each breath out. It works great and after about 5 minutes I'm usually pretty numb and unaware of my body and enjoying expanded consciousness. After a while, I'm done and I decide to roll over and go to sleep.

It's absolutely impossible, my mind isn't racing as such but I just can't sleep and feel a bit messed up/caffeinated.

Thinking back, this is what used to happen every time I did this exercise and which is why I never slept that well in the past when I did it.

This is just a simple relaxation exercise that should, if anything, aid sleep. What state is it putting my brain in that makes it impossible to sleep for 1-1.5h afterwards? Any ideas?