I WBTB quite a few times throughout the night. WBTB is good for increasing how present you are in a dream, dream and your recall, but without some mild or anything it will have little effect. I would recommend having short WBTBs throughout the night and a long one later in the night. Short being about 5 minutes. Long being about 15-20 minutes. If you are behind on sleep, I wouldn't recommend the long one, because getting enough sleep is better than doing well WBTB. Instead of WBTB being something that you try, try making l a form of WBTB that you can do each night.