Sounds like you are breaking up your delta sleep too much, but depends when you are waking?

Normally if you wake after about 5 hours for WBTB you should be okay, but wouldn't want to do this everyday.

When I was experementing with a technique i was putting together I found that after 4 solid days of getting up after 4-5 hours I was getting tired because I was breaking up my delta (deep) sleep - But after 5 hours was easier to manage.

I will say though I also noticed that doing it so often in a row, meant that I needed more deep sleep to recover meaning less REM sleep, which could impact lucid dreams.

Adam.