I am by no means an expert but as far as i know:

When you go to sleep at night you will first go through a deep sleep phase and after this you will enter REM cycles every 90 minutes or so and during this deep sleep your brain activity drops dramatically.

When you nap in the middle of the day, you won't fall into deep sleep, but instead directly into REM thus making it easier to WILD.

Also, the WBTB technique is effective to help induce lucid dreams because as you wake up in the night, you will regain the awareness lost during deep sleep, thus giving you higher awareness when you go to sleep again since you won't go into deep sleep.

That is why it is used for WILD. To get past deep sleep.