The short version is that the authors found that sometimes we really do think too much and when we do, out biases interfere with our knowledge and ability and our decision making actually gets worse. On the other hand sleeping (and I would venture yoga, meditation, and exercise) seems to clear the brain so that it (we) can make a better decision.
Why Sleeping On It Helps | World of Psychology:
If this study is right, you can think yourself into the wrong decision.
"...to emphasize this finding — if you’re an expert and you had extra time to think about your decision in the area of your expertise (conscious thinker) or had to make a quick decision, you made worse decisions than those who were unconscious thinkers. The researcher hypothesize that conscious thought can lead to poor weighting in decision-making — the more you think about something, the more your biases interfere with good decision-making.
Unconscious thinkers in this experiment appear to weight the relative importance of diagnostic information more accurately than conscious thinkers did."