It's visualization. Years ago I read the book "Creative visualization" by Shakti Gawain, now I reread it. (Btw, the book is great.)
I remembered this book, because visualization happened spontaneously when I went to bed before sleeping. Obviously I did the exercise at the wrong time or was it only the wrong pose that I imagined? Laying in bed on my back I imagined how I arched back from standing position to do urdhva dhanurasana. Also in my imaginations I fell back suddenly and exactly at the moment when I fell down, I woke up from my very relaxed almost sleeping state. The consequence: I had difficulties to sleep in. Yesterday when I was in bed, I thought it could be better to imagine myself pressing me up from the floor. This is so exhausting that I must sleep quickly. I must have slept in quickly as I cannot remember anymore what happened. But to sleep in quickly was not my intention when I wanted to do visualization. I wanted to improve some of the asanas that way.
The consequence: Visualization can be done any time. In order to get the most out of it without creating sleeping in problems it's good to reserve some extra time during the day.